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().

MFC after: 1 weeek
Differential Revision: https://reviews.freebsd.org/D28647