RDMA/core: Define client_data_lock as rwlock instead of spinlock

Even though device registration/unregistration and client
registration/unregistration is not a performance path, define the
client_data_lock as rwlock for code clarity.

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