net: IP_MULTICAST_IF setsockopt now recognizes struct mreq

Until now, struct mreq has not been recognized and it was worked with
as with struct in_addr. That means imr_multiaddr was copied to
imr_address. So do recognize struct mreq here and copy that correctly.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>