null_blk: fix return value from null_add_dev()

The function nullb_device_power_store() returns -ENOMEM when
null_add_dev() fails. null_add_dev() can fail with return value
other than -ENOMEM such as -EINVAL when Zoned Block Device option
is used, see :

nullb_device_power_store()
 null_add_dev()
  null_init_zoned_dev()
	return -EINVAL;

When trying to load the module having -ENOMEM value returned on the
command line creates confusion when pleanty of memory is free on the
machine.

Instead of hardcoding -ENOMEM return the value of null_add_dev()
function.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220215115951.15945-1-kch@nvidia.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>