netinet6: ip6_setpktopt() requires NET_EPOCH

ip6_setpktopt() can call ifnet_byindex() which requires epoch. Mark the
function as requiring NET_EPOCH, and ensure we enter it priot to calling
it.

Reported-by: syzbot+92526116441688fea8a3@syzkaller.appspotmail.com
Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33462