md: ensure new badblocks are handled promptly.

When we mark blocks as bad we need them to be acknowledged by the
metadata handler promptly.

For an in-kernel metadata handler that was already being done.  But
for an external metadata handler we need to alert it of the change by
sending a notification through the sysfs file.  This adds that
notification.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -7858,6 +7858,7 @@ int rdev_set_badblocks(struct md_rdev *rdev, sector_t s, int sectors,
 				  s + rdev->data_offset, sectors, acknowledged);
 	if (rv) {
 		/* Make sure they get written out promptly */
+		sysfs_notify_dirent_safe(rdev->sysfs_state);
 		set_bit(MD_CHANGE_CLEAN, &rdev->mddev->flags);
 		md_wakeup_thread(rdev->mddev->thread);
 	}