umem can be NULL for user application mkeys in some cases. Therefore
umem can't be used for checking if the mkey is cacheable and it is
changed for checking a flag that indicates it. Also make sure that
all mkeys which are not returned to the cache will be destroyed.

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/2690bc5c6896bcb937f89af16a1ff0343a7ab3d0.1712140377.git.leon@kernel.orgSigned-off-by: Leon Romanovsky <leon@kernel.org>