return ifnet double pointer.
Pass that hint down to in6_selectif() to be used when i) the default FIB
is queried and ii) route lookup fails because the network is not present
(i.e. someone deleted the connected subnet).
This hint should not be generally used from anywhere outside the neighbor
discovery code. We just make use of it from nd6_ns_output().
Extend the nd6_na_output() interface by a nd6_na_output_fib() version
and pass the FIB number from the NS mbuf on to NA to allow the new mbuf
to inherit the FIB tag and a later lookup from ip6_output() to succeed
in the aformentioned example case.
Provide a wrapper function for the old public interface also used from
CARP but mark it with BURN_BRIDGES to cleanup in HEAD after MFC.
Sponsored by: Cisco Systems, Inc.
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