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
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.
Note: depends on D28647.