Do not reference returned ifa in in6_ifawithifp().

The only place where in6_ifawithifp() is used is ip6_output(),
which uses the returned ifa to bump traffic counters.
Given ifa stability guarantees is provided by epoch, do not refcount ifa.

This eliminates 2 atomic ops from IPv6 fast path.

Reviewed By: rstone
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28649