The Infiniband spec defines "A multicast address is defined by a
MGID and a MLID" (section 10.5).

The current code only uses the MGID for identifying multicast groups.
Update the driver to be compliant with this definition.
Reviewed-by: NIra Weiny <ira.weiny@intel.com>
Reviewed-by: NDasaratharaman Chandramouli <dasaratharaman.chandramouli@intel.com>
Signed-off-by: NMichael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: NDennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>