drbd: Do not upgrade state to Outdated if already Inconsistent [Bugz 277]

There was a race condition:
  In a situation with a SyncSource+Primary and a SyncTarget+Secondary node,
  and a resync dependency to some other device. After both nodes decided
  to do the resync, the other device finishes its resync process.
  At that time SyncSource already sent the P_SYNC_UUID packet, and
  already updated its peer disk state to Inconsistent.
  The SyncTarget node waits for the P_SYNC_UUID and sends a state packet
  to report the resync dependency change. That packet still carries
  a disk state of Outdated.

Impact:
  If application writes come in, during that time on the Primary node,
  those do not get replicated, and the out-of-sync counter gets increased.
  => The completion of resync is not detected on the primary node.
  => stalled.
  Those blocks get resync'ed with the next resync, since the are get
  marked as out-of-sync in the bitmap.

In order to fix this, we filter out that wrong state change in the
sanitize_state() function.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_main.c

@@ -840,7 +840,7 @@ static union drbd_state sanitize_state(struct drbd_conf *mdev, union drbd_state
 			break;
 		case C_WF_BITMAP_S:
 		case C_PAUSED_SYNC_S:
-			ns.pdsk = D_OUTDATED;
+			ns.pdsk = os.pdsk > D_OUTDATED ? D_OUTDATED : os.pdsk;
 			break;
 		case C_SYNC_SOURCE:
 			ns.pdsk = D_INCONSISTENT;