IB/cm: Use sgid_attr from the AV

Prior patches now ensure that the AV has a sgid_attr, if one would have
been required.  Instead of querying for one, take it directly from the AH.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>