RDMA/cm: Use refcount_t type for refcount variable

This atomic in struct cm_id_private is being used as a refcount, change it
to refcount_t for better clarity and to get the refcount protections.

Link: https://lore.kernel.org/r/1573997601-4502-1-git-send-email-danitg@mellanox.com


Signed-off-by: Danit Goldberg <danitg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>