This gets us support for non-discard efficient write of zeroes (e.g. NVMe)
and prepares for removing the discard_zeroes_data flag.

Also remove a pointless discard support check, which is done in
blkdev_issue_discard already.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>