Enforce net epoch in in6_selectsrc().

in6_selectsrc() may call fib6_lookup() in some cases, which requires
epoch. Wrap in6_selectsrc* calls into epoch inside its users.
Mark it as requiring epoch by adding NET_EPOCH_ASSERT().

Differential Revision: https://reviews.freebsd.org/D28647

(cherry picked from commit 605284b894748d23136b30a202689493d8f8af52)