block: introduce BLKDEV_DISCARD_ZERO to fix zeroout

Currently blkdev_issue_zeroout cascades down from discards (if the driver
guarantees that discards zero data), to WRITE SAME and then to a loop
writing zeroes.  Unfortunately we ignore run-time EOPNOTSUPP errors in the
block layer blkdev_issue_discard helper to work around DM volumes that
may have mixed discard support underneath.

This patch intoroduces a new BLKDEV_DISCARD_ZERO flag to
blkdev_issue_discard that indicates we are called for zeroing operation.
This allows both to ignore the EOPNOTSUPP hack and actually consolidating
the discard_zeroes_data check into the function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>