RDMA/cma: Use refcount API to reflect refcount

Use the refcount variant to capture the reference counting of the cma
device structure.

Link: https://lore.kernel.org/r/20200126142652.104803-6-leon@kernel.org


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