inet6: Do not assume every interface has ip6 enabled.

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
Reviewed by: melifaro, hrs
Differential Revision: https://reviews.freebsd.org/D4522