block: stop using discards for zeroing

Now that we have REQ_OP_WRITE_ZEROES implemented for all devices that
support efficient zeroing, we can remove the call to blkdev_issue_discard.
This means we only have two ways of zeroing left and can simplify the
code.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Martin K. Petersen <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-lib.c

···
  *  Zero-fill a block range, either using hardware offload or by explicitly
  *  writing zeroes to the device.
  *
+ *  Note that this function may fail with -EOPNOTSUPP if the driver signals
+ *  zeroing offload support, but the device fails to process the command (for
+ *  some devices there is no non-destructive way to verify whether this
+ *  operation is actually supported).  In this case the caller should call
+ *  retry the call to blkdev_issue_zeroout() and the fallback path will be used.
+ *
  *  If a device is using logical block provisioning, the underlying space will
  *  not be released if %flags contains BLKDEV_ZERO_NOUNMAP.
  *
···
 	int ret;
 	struct bio *bio = NULL;
 	struct blk_plug plug;
-
-	if (!(flags & BLKDEV_ZERO_NOUNMAP)) {
-		if (!blkdev_issue_discard(bdev, sector, nr_sects, gfp_mask,
-				BLKDEV_DISCARD_ZERO))
-			return 0;
-	}
 
 	blk_start_plug(&plug);
 	ret = __blkdev_issue_zeroout(bdev, sector, nr_sects, gfp_mask,