RDMA/mlx5: Check reg_create() create for errors

The reg_create() can fail.  Check for errors before dereferencing it.

Fixes: dd1b913f ("RDMA/mlx5: Cache all user cacheable mkeys on dereg MR flow")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/Y+ERYy4wN0LsKsm+@kili


Reviewed-by: Devesh Sharma <devesh.s.sharma@oracle.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>