inet6: Do not assume every interface has ip6 enabled

[kp]

Summary

Certain interfaces (e.g. pfsync0) do not have ip6 addresses (in other words,
ifp->if_afdata[AF_INET6] is NULL). Ensure we don't panic when the MTU is
updated.

pfsync interfaces will never have ip6 support, because it's explicitly disabled
in in6_domifattach().

PR: 205194

[melifaro]

According to the summary, it looks ok. Ensuring entire inet6/ code would work in all similar cases require (probably much) more work :)

sys/netinet6/in6.c

2427

Well, the function is supposed to return MTU value. 46 is definitely better than panic, but It would probably be better to return ifp->if_mtu.

[kp]

sys/netinet6/in6.c

2427

You're quite right. We should do what the caller (if_getmtu_family()) does if it can't get a better answer and return ifp->if_mtu.