Enter the net epoch before calling ip6_setpktopts

ip6_setpktopts() can look up ifnets via ifnet_by_index(), which
is only safe in the net epoch. Ensure that callers are in the net
epoch before calling this function.

Sponsored by: Dell EMC Isilon
MFC after: 4 weeks
Reviewed by: donner, kp
Differential Revision: https://reviews.freebsd.org/D30630