nvmet: avoid double errno conversions

Use errno_to_nvme_status to convert from a negative errno to a
nvme status field instead of going through a blk_status_t.

Also remove the pointless status variable in
nvmet_bdev_execute_write_zeroes.

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