mlxsw: spectrum_span: Allow bridge for gretap mirror

When handling mirroring to a gretap or ip6gretap netdevice in mlxsw, the
underlay address (i.e. the remote address of the tunnel) may be routed
to a bridge.

In that case, look up the resolved neighbor Ethernet address in that
bridge's FDB. Then configure the offload to direct the mirrored traffic
to that port, possibly with tagging.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>