drbd: Implemented two new connection states Ahead/Behind

In this connection mode, the ahead node no longer replicates
application IO. The behind's disk becomes out dated.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

include/linux/drbd.h

@@ -206,6 +206,10 @@ enum drbd_conns {
 	C_VERIFY_T,
 	C_PAUSED_SYNC_S,
 	C_PAUSED_SYNC_T,
+
+	C_AHEAD,
+	C_BEHIND,
+
 	C_MASK = 31
 };