ethernet: Add helpers to recognize addresses mapped to IP multicast

IP multicast must sometimes be discriminated from non-IP multicast,
e.g. when determining the forwarding behavior of a given group in the
presence of multicast router ports on an offloaded bridge. Therefore,
provide helpers to identify these groups.

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>