RDMA/mlx5: Rename the tracking variables for the MR cache

The old names do not clearly indicate the intent.

Link: https://lore.kernel.org/r/20200310082238.239865-6-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>