md: restore ability of spare drives to spin down.

Some time ago we stopped the clean/active metadata updates
from being written to a 'spare' device in most cases so that
it could spin down and say spun down.  Device failure/removal
etc are still recorded on spares.

However commit 51d5668c broke this 50% of the time,
depending on whether the event count is even or odd.
The change log entry said:

   This means that the alignment between 'odd/even' and
    'clean/dirty' might take a little longer to attain,

how ever the code makes no attempt to create that alignment, so it
could take arbitrarily long.

So when we find that clean/dirty is not aligned with odd/even,
force a second metadata-update immediately.  There are already cases
where a second metadata-update is needed immediately (e.g. when a
device fails during the metadata update).  We just piggy-back on that.
Reported-by: NJoe Bryant <tenminjoe@yahoo.com>
Signed-off-by: NNeilBrown <neilb@suse.de>
Cc: stable@kernel.org

drivers/md/md.c

@@ -2109,12 +2109,18 @@ static void md_update_sb(mddev_t * mddev, int force_change)
 		if (!mddev->in_sync || mddev->recovery_cp != MaxSector) { /* not clean */
 			/* .. if the array isn't clean, an 'even' event must also go
 			 * to spares. */
-			if ((mddev->events&1)==0)
+			if ((mddev->events&1)==0) {
 				nospares = 0;
+				sync_req = 2; /* force a second update to get the
+					       * even/odd in sync */
+			}
 		} else {
 			/* otherwise an 'odd' event must go to spares */
-			if ((mddev->events&1))
+			if ((mddev->events&1)) {
 				nospares = 0;
+				sync_req = 2; /* force a second update to get the
+					       * even/odd in sync */
+			}
 		}
 	}