nvmet: return a specified error it subsys_alloc fails

nvmet_subsys_alloc() returns its pointer or NULL if it fails.  We can
see three different steps in this function:
  1. memory allocation
  2. argument check
  3. memory allocation for string

But now the callers of this function do not seem to handle case 2 by
returning -ENOMEM only even if it fails with an invalid parameter.

This patch specifies error codes so that caller can pass it to its own
caller.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com&gt;.>
Signed-off-by: Christoph Hellwig <hch@lst.de>