compat/linux: fix IFLA_IFNAME translation for multi-message netlink

[pouria]

Summary

nlmsg_translate_ifname_nla() always used nw->ifp for the name,
which is fine for a single-message ifnet event, but an RTM_GETLINK
dump holds one RTM_NEWLINK per interface and is translated with
the ifp the writer had when the buffer was flushed.

The root of the problem is that msgs_to_linux() takes a single
ifp for a buffer that may contain messages about many interfaces.

Use nw->ifp only when its index matches the ifi_index, otherwise
resolve the name by index.

Assisted by: Claude (Fable 5.1)

*Disclosure*:
I'm not an expert in compat/linux, but I know how our netlink works.
I used an *AI* assistant to analyze linux_netlink.c and draft the initial diff after I had narrowed the problem to the kernel side with strace.
I reviewed and reworked the patch and tested it myself (dump, single reply, up/down events and interface removal), and I take full responsibility for it.

Test Plan

On a machine with several interfaces enough for the dump to span more than one flush:

Dump: chroot /compat/ubuntu ip link show
Before:

# chroot /compat/ubuntu ip link sh                                                                                                                                             
1: epair0a: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000                                         
    link/ether 58:9c:fc:04:db:5c brd ff:ff:ff:ff:ff:ff          
2: epair0a: <LOOPBACK,MULTICAST,UP,LOWER_UP> mtu 16384 qdisc noqueue state UP qlen 1000                                         
    link/loopback                                                       
3: epair0a: <> mtu 1500 qdisc noqueue state DOWN qlen 1000      
    link/[247]                                                     
4: epair0a: <> mtu 1536 qdisc noqueue state DOWN qlen 1000                                                                      
    link/[244]                                                  
5: epair0a: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000                                         
    link/ether 58:9c:fc:10:31:1e brd ff:ff:ff:ff:ff:ff                                                                          
6: bridge0: <BROADCAST,MULTICAST,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000                                            
    link/ether 58:9c:fc:10:dc:2a brd ff:ff:ff:ff:ff:ff          
7: bridge0: <BROADCAST,MULTICAST,PROMISC,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000                                    
    link/ether 58:9c:fc:10:c0:8d brd ff:ff:ff:ff:ff:ff                                                                                                                                          
8: bridge0: <BROADCAST,MULTICAST,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000                                                                                                            
    link/ether 58:9c:fc:10:60:45 brd ff:ff:ff:ff:ff:ff                                                                          
9: bridge0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000                                         
    link/ether 58:9c:fc:10:e9:85 brd ff:ff:ff:ff:ff:ff

After:

# chroot /compat/ubuntu ip link sh
1: vtnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 58:9c:fc:04:db:5c brd ff:ff:ff:ff:ff:ff
2: lo: <LOOPBACK,MULTICAST,UP,LOWER_UP> mtu 16384 qdisc noqueue state UP qlen 1000
    link/loopback
3: pfsync0: <> mtu 1500 qdisc noqueue state DOWN qlen 1000
    link/[247]
4: enc0: <> mtu 1536 qdisc noqueue state DOWN qlen 1000
    link/[244]
5: epair0a: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 58:9c:fc:10:31:1e brd ff:ff:ff:ff:ff:ff
6: epair0b: <BROADCAST,MULTICAST,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 58:9c:fc:10:dc:2a brd ff:ff:ff:ff:ff:ff
7: epair1a: <BROADCAST,MULTICAST,PROMISC,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 58:9c:fc:10:c0:8d brd ff:ff:ff:ff:ff:ff
8: epair1b: <BROADCAST,MULTICAST,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 58:9c:fc:10:60:45 brd ff:ff:ff:ff:ff:ff
9: bridge0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP qlen 1000
    link/ether 58:9c:fc:10:e9:85 brd ff:ff:ff:ff:ff:ff

Single reply: chroot /compat/ubuntu ip link show dev vtnet0
Events: chroot /compat/ubuntu ip monitor link while running:
ifconfig epair0a down; ifconfig epair0a up

Removal: chroot /compat/ubuntu ip monitor link while running:
ifconfig epair0a destroy

Repeated 1 with compat.linux.use_real_ifnames=0 and 1

[pouria]

sys/compat/linux/linux_netlink.c

258

The __DECONST part is ugly, but it was the only semi-clean way that I could think of