md: notify the 'degraded' sysfs attribute on failure.

We currently only 'notify' changes to the 'degraded' attribute
when it decreases, not when it increases.

Notifying on failure is a little awkward as it happen in
interrupt context.
So instead, notify when we remove the failed device from the array,
which is very soon afterwards.
Reported-and-tested-by: Mikhail Balabin <mbalabin@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -7383,6 +7383,7 @@ static int remove_and_add_spares(struct mddev *mddev)
 {
 	struct md_rdev *rdev;
 	int spares = 0;
+	int removed = 0;
 
 	mddev->curr_resync_completed = 0;
 
@@ -7396,8 +7397,13 @@ static int remove_and_add_spares(struct mddev *mddev)
 				    mddev, rdev) == 0) {
 				sysfs_unlink_rdev(mddev, rdev);
 				rdev->raid_disk = -1;
+				removed++;
 			}
 		}
+	if (removed)
+		sysfs_notify(&mddev->kobj, NULL,
+			     "degraded");
+
 
 	list_for_each_entry(rdev, &mddev->disks, same_set) {
 		if (rdev->raid_disk >= 0 &&