nvmet-file: fix nvmet_file_flush() always returning an error

Presently, nvmet_file_flush() always returns a call to
errno_to_nvme_status() but that helper doesn't take into account the
case when errno=0. So nvmet_file_flush() always returns an error code.

All other callers of errno_to_nvme_status() check for success before
calling it.

To fix this, ensure errno_to_nvme_status() returns success if the
errno is zero. This should prevent future mistakes like this from
happening.

Fixes: c6aa3542

 ("nvmet: add error log support for file backend")
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>