md-cluster: wakeup thread if activated a spare disk

When a device is re-added, it will ultimately need
to be activated and that happens in md_check_recovery,
so we need to set MD_RECOVERY_NEEDED right after
remove_and_add_spares.

A specifical issue without the change is that when
one node perform fail/remove/readd on a disk, but
slave nodes could not add the disk back to array as
expected (added as missed instead of in sync). So
give slave nodes a chance to do resync.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -8694,6 +8694,11 @@ static void check_sb_changes(struct mddev *mddev, struct md_rdev *rdev)
 				ret = remove_and_add_spares(mddev, rdev2);
 				pr_info("Activated spare: %s\n",
 						bdevname(rdev2->bdev,b));
+				/* wakeup mddev->thread here, so array could
+				 * perform resync with the new activated disk */
+				set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
+				md_wakeup_thread(mddev->thread);
+
 			}
 			/* device faulty
 			 * We just want to do the minimum to mark the disk