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
Differential Revision: https://reviews.freebsd.org/D28649

(cherry picked from commit 1bd44b11e59f1e9ee7245f8de1f823bc5287b9ef)