block: nvme-scsi: Catch kcalloc failure

res variable was initialized to -ENOMEM, but it's override by
nvme_trans_copy_from_user(). So current code returns 0 if kcalloc fails.
Fix it to return proper error code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Jens Axboe <axboe@fb.com>