nvmet: fix file discard return status

If nvmet_copy_from_sgl failed, we falsly return successful
completion status.

Fixes: d5eff33e ("nvmet: add simple file backed ns support")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>