Remove per-packet ifa refcounting from IPv6 fast path.

Currently ip6_input() calls in6ifa_ifwithaddr() for
every local packet, in order to check if the target ip
belongs to the local ifa in proper state and increase
its counters.

in6ifa_ifwithaddr() references found ifa.
With epoch changes, both ip6_input() and all other current callers
of in6ifa_ifwithaddr() do not need this reference
anymore, as epoch provides stability guarantee.

Given that, update in6ifa_ifwithaddr() to allow
it to return ifa without referencing it, while preserving
option for getting referenced ifa if so desired.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28648