RDMA/mlx5: Store in the cache mkeys instead of mrs

Currently, the driver stores mlx5_ib_mr struct in the cache entries,
although the only use of the cached MR is the mkey. Store only the mkey in
the cache.

Link: https://lore.kernel.org/r/20220726071911.122765-5-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>