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: c6aa354 ("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>

drivers/nvme/target/core.c

@@ -46,6 +46,9 @@ inline u16 errno_to_nvme_status(struct nvmet_req *req, int errno)
  u16 status;
 
  switch (errno) {
+ case 0:
+ status = NVME_SC_SUCCESS;
+ break;
  case -ENOSPC:
  req->error_loc = offsetof(struct nvme_rw_command, length);
  status = NVME_SC_CAP_EXCEEDED | NVME_SC_DNR;