RDMA/mlx5: Rename the mkey cache variables and functions

After replacing the MR cache with an Mkey cache, rename the variables and
functions to fit the new meaning.

Link: https://lore.kernel.org/r/20220726071911.122765-6-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>