block: ensure discard_granularity is zero when discard is not supported

Documentation/ABI/stable/sysfs-block states:

  What: /sys/block/<disk>/queue/discard_granularity
  [...]
  A discard_granularity of 0 means that the device does not support
  discard functionality.

but this got broken when sorting out the block limits updates.  Fix this
by setting the discard_granularity limit to zero when the combined
max_discard_sectors is zero.

Fixes: 3c407dc ("block: default the discard granularity to sector size")
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Martin K. Petersen <[email protected]>

Author: Christoph Hellwig

block/blk-settings.c

@@ -388,12 +388,19 @@ int blk_validate_limits(struct queue_limits *lim)
 	lim->max_discard_sectors =
 		min(lim->max_hw_discard_sectors, lim->max_user_discard_sectors);
 
+	/*
+	 * When discard is not supported, discard_granularity should be reported
+	 * as 0 to userspace.
+	 */
+	if (lim->max_discard_sectors)
+		lim->discard_granularity =
+			max(lim->discard_granularity, lim->physical_block_size);
+	else
+		lim->discard_granularity = 0;
+
 	if (!lim->max_discard_segments)
 		lim->max_discard_segments = 1;
 
-	if (lim->discard_granularity < lim->physical_block_size)
-		lim->discard_granularity = lim->physical_block_size;
-
 	/*
 	 * By default there is no limit on the segment boundary alignment,
 	 * but if there is one it can't be smaller than the page size as