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: b9358bdb ("RDMA/mlx5: Fix locking in MR cache work queue")
Link: https://lore.kernel.org/r/c99f076fce4b44829d434936bbcd3b5fc4c95020.1649062436.git.leonro@nvidia.comSigned-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>

drivers/infiniband/hw/mlx5/mr.c

@@ -627,6 +627,7 @@ static void mlx5_mr_cache_free(struct mlx5_ib_dev *dev, struct mlx5_ib_mr *mr)
 {
 	struct mlx5_cache_ent *ent = mr->cache_ent;
 
+	WRITE_ONCE(dev->cache.last_add, jiffies);
 	spin_lock_irq(&ent->lock);
 	list_add_tail(&mr->list, &ent->head);
 	ent->available_mrs++;