RDMA/mlx5: Uncacheable mkey has neither rb_key or cache_ent

As some mkeys can't be modified with UMR due to some UMR limitations,
like the size of translation that can be updated, not all user mkeys can
be cached.

Fixes: dd1b913 ("RDMA/mlx5: Cache all user cacheable mkeys on dereg MR flow")
Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Link: https://lore.kernel.org/r/f2742dd934ed73b2d32c66afb8e91b823063880c.1712140377.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/hw/mlx5/mlx5_ib.h

@@ -643,7 +643,7 @@ struct mlx5_ib_mkey {
 	unsigned int ndescs;
 	struct wait_queue_head wait;
 	refcount_t usecount;
-	/* User Mkey must hold either a rb_key or a cache_ent. */
+	/* Cacheable user Mkey must hold either a rb_key or a cache_ent. */
 	struct mlx5r_cache_rb_key rb_key;
 	struct mlx5_cache_ent *cache_ent;
 };