RDMA/mlx5: Add a missing update of cache->last_add

RDMA/mlx5: Add a missing update of cache->last_add

Update cache->last_add when returning an MR to the cache so that the cache
work won't remove it.

Fixes: b9358bdbc713 ("RDMA/mlx5: Fix locking in MR cache work queue")
Link: https://lore.kernel.org/r/c99f076fce4b44829d434936bbcd3b5fc4c95020.1649062436.git.leonro@nvidia.com
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>