RDMA/ucma: Change backlog into an atomic

There is no reason to grab the file->mut just to do this inc/dec work. Use
an atomic.

Link: https://lore.kernel.org/r/20200818120526.702120-12-leon@kernel.org


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