mlxsw: spectrum_span: Allow bridge for gretap mirror

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 <[email protected]>
Signed-off-by: Ido Schimmel <[email protected]>
Reviewed-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>