Fix mbuf leak in NDP

[smalukav_gmail.com]

Summary

Incomplete records may contain packets in their hold queue - https://reviews.freebsd.org/source/src/browse/main/sys/net/if_llatbl.h;89e58b955cf5d2187af5f2460d11b64f4d229c8a$69. When you remove an incomplete record, its packets should be freed in
lltable_drop_entry_queue - https://reviews.freebsd.org/source/src/browse/main/sys/net/if_llatbl.c;89e58b955cf5d2187af5f2460d11b64f4d229c8a$286. But NDP code never increases la_numheld, and the removal doesn't happen.

To fix I make a common function to add a packet to the record's hold queue - lltable_append_entry_queue. I use this function in ARP and NDP code instead of their custom implementations.

[melifaro]

LGTM! Put a number of non-critical comments. Should be good to land once addressed.

sys/net/if_llatbl.c

141

Worth adding WLOCK_ASSERT here

143

Could you declare next here?

151

Could you declare cure here?

322–323

sys/netinet/icmp6.h

605

s/reply/resolution/ ?

sys/netinet/if_ether.c

542–543

I'd really prefer to split actual work and counter increment. If someone decides (for whatever reason) to do something with the counters, including things like define ARPSTAT_ADD, this will break in a weird way.
Could you make it 2 lines?

sys/netinet6/nd6.c

809

2488–2489

Could you split it into 2 lines?

usr.bin/netstat/inet6.c

998