rip6_output not in net epoch across call to ip6_setpktopts()

[Ryan Stone]

Recently at work we saw a panic in ip6_setpktopt() while it's using an
ifnet returned by ifnet_byindex, here:

https://cgit.freebsd.org/src/tree/sys/netinet6/ip6_output.c#n2969

The issue seems to be that rip6_output() calls into ip6_setpktopts()
outside of the net epoch.  Should I just wrap the setpktopts call in a
net epoch enter/exit, or does anybody think that there's something
cleverer that should be done there?