RDMA/ucma: Use refcount_t for the ctx->ref

Don't use an atomic as a refcount.

Link: https://lore.kernel.org/r/20200218191657.GA29724@ziepe.ca


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