md: fix bug in raid10 recovery.

Adding a spare to a raid10 doesn't cause recovery to start.
This is due to an silly type in
  commit 6c2fce2ef6b4821c21b5c42c7207cb9cf8c87eda
and so is a bug in 2.6.27 and .28-rc.

Thanks to Thomas Backlund for bisecting to find this.

Cc: Thomas Backlund <tmb@mandriva.org>
Cc: stable@kernel.org

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -1137,7 +1137,7 @@ static int raid10_add_disk(mddev_t *mddev, mdk_rdev_t *rdev)
 	if (!enough(conf))
 		return -EINVAL;
 
-	if (rdev->raid_disk)
+	if (rdev->raid_disk >= 0)
 		first = last = rdev->raid_disk;
 
 	if (rdev->saved_raid_disk >= 0 &&