RDMA/nldev: missing error code in nldev_res_get_doit()

We should return -ENOMEM if the allocation fails.  The current code
accidentally returns success.

Fixes: bf3c5a93 ("RDMA/nldev: Provide global resource utilization")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>