RDMA/cma: Use rdma_read_gid_attr_ndev_rcu to access netdev

To access the netdevice of the GID attribute, use an existing API
rdma_read_gid_attr_ndev_rcu().

This further reduces dependency on open access to netdevice of GID
attribute.

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