RDMA/mlx5: Store the number of in_use cache mkeys instead of total_mrs

total_mrs is used only to calculate the number of mkeys currently in
use. To simplify things, replace it with a new member called "in_use" and
directly store the number of mkeys currently in use.

Link: https://lore.kernel.org/r/20220726071911.122765-4-michaelgur@nvidia.com


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