RDMA/mlx5: Uncacheable mkey has neither rb_key or cache_ent

RDMA/mlx5: Uncacheable mkey has neither rb_key or cache_ent

[ Upstream commit 0611a8e8b475fc5230b9a24d29c8397aaab20b63 ]

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: dd1b913fb0d0 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>