RDMA/mlx5: Change check for cacheable mkeys

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.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>