nvme.h: add Write Zeroes definitions

Add the command structure, optional command set support (ONCS) bit and
a new error code for the Write Zeroes command.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@hgst.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>