RDMA: Use GID from the ib_gid_attr during the add_gid() callback

Now that ib_gid_attr contains the GID, make use of that in the add_gid()
callback functions for the provider drivers to simplify the add_gid()
implementations.

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