nvmet: add error log support for file backend

This patch adds support for the file backend to populate the
error log entries. Here we map the errno to the NVMe status codes.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>