inet6: protect address manipulation with a lock

[mjg]

Summary

inet6: protect address manipulation with a lock

This is a total hack/bare minimum which follows inet4.

Otherwise 2 threads removing the same address can easily crash.

Test Plan

#!/bin/sh

IFNAME=igc0
INET6ADDR=2001:db8:bdbd::123/48

while true; do
    ifconfig $IFNAME inet6 $INET6ADDR &                      
    ifconfig $IFNAME inet6 $INET6ADDR delete &                   
done

crashes stock kernel with:

--- trap 0xc, rip = 0xffffffff80fd9293, rsp = 0xfffffe00cd68c9d0, rbp = 0xfffffe00cd68ca20 ---
in6_unlink_ifa() at in6_unlink_ifa+0x63/frame 0xfffffe00cd68ca20
in6_purgeaddr() at in6_purgeaddr+0x367/frame 0xfffffe00cd68cb40
in6_purgeifaddr() at in6_purgeifaddr+0x13/frame 0xfffffe00cd68cb60
in6_control() at in6_control+0x532/frame 0xfffffe00cd68cbc0
ifioctl() at ifioctl+0x7bc/frame 0xfffffe00cd68ccc0
kern_ioctl() at kern_ioctl+0x26d/frame 0xfffffe00cd68cd30
sys_ioctl() at sys_ioctl+0x101/frame 0xfffffe00cd68ce00
amd64_syscall() at amd64_syscall+0x10c/frame 0xfffffe00cd68cf30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe00cd68cf30

[kp]

fwiw, Gleb added the IPv4 lock in https://cgit.freebsd.org/src/commit/?id=f7a39160c259ea83b507b0808418619712572428.

It's a bit of a big hammer, but it's going to be more effective than finding and handling individual races, and it's probably not very important to be able to add/remove millions of addresses per second.

Nit: it should be a per-vnet lock, but the IPv4 one isn't either. Perhaps that's something we should fix in a separate commit.

[zlei]

Xref a PR: 271344 which was reported after the fix.