block: don't try to discard from __blkdev_issue_zeroout

Discard can return -EIO asynchronously if the alignment for the request
isn't suitable for the driver, which makes a proper fallback to other
methods in __blkdev_issue_zeroout impossible.  Thus only issue a sync
discard from blkdev_issue_zeroout an don't try discard at all from
__blkdev_issue_zeroout as a non-invasive workaround.

One more reason why abusing discard for zeroing must die..

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Eryu Guan <eguan@redhat.com>
Fixes: e73c23ff ("block: add async variant of blkdev_issue_zeroout")
Signed-off-by: Jens Axboe <axboe@fb.com>