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: dd1b913f

 ("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>