This is really just a CM support function, normally a multicast address
does not have a specific SGID - but the RDMA CM usage model does restrict
things to the netdevice the CM id is bound to, at least for roce case.

Store the selected table entry in the sgid_attr for everything else to
use.
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>