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/[e-mail]
Signed-off-by: Aharon Landau <[e-mail]>
Reviewed-by: Shay Drory <[e-mail]>
Signed-off-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/hw/mlx5/mr.c

···
 {
 	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++;