drbd: fix for spurious full sync (becoming sync target looked like invalidate)

If a synctarget lost connection while being WFSyncUUID,
due to "state sanitizing", the attempted state change to SyncTarget
looked like an "invalidate" to after_state_ch() later,
thus caused a full sync on next handshake (Bug #318).

drbd0: PingAck did not arrive in time.
drbd0: peer( Primary -> Unknown ) conn( WFSyncUUID -> NetworkFailure ) pdsk( UpToDate -> DUnknown )

        from  : { cs:NetworkFailure ro:Secondary/Unknown ds:UpToDate/DUnknown r--- }
        to    : { cs:SyncTarget ro:Secondary/Unknown ds:Inconsistent/DUnknown r--- }
        after sanizising, resulted in
        state: { cs:NetworkFailure ro:Secondary/Unknown ds:Inconsistent/DUnknown r--- }
        drbd0: disk( UpToDate -> Inconsistent )

Fix:
don't mask state transition errors in "sanitizing",
so the requested state change to SyncTarget fails,
instead of being implicitly "remaped" to invalidate.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_main.c

@@ -827,9 +827,10 @@ static union drbd_state sanitize_state(struct drbd_conf *mdev, union drbd_state
 	    os.conn <= C_DISCONNECTING)
 		ns.conn = os.conn;
 
-	/* After a network error (+C_TEAR_DOWN) only C_UNCONNECTED or C_DISCONNECTING can follow */
+	/* After a network error (+C_TEAR_DOWN) only C_UNCONNECTED or C_DISCONNECTING can follow.
+	 * If you try to go into some Sync* state, that shall fail (elsewhere). */
 	if (os.conn >= C_TIMEOUT && os.conn <= C_TEAR_DOWN &&
-	    ns.conn != C_UNCONNECTED && ns.conn != C_DISCONNECTING)
+	    ns.conn != C_UNCONNECTED && ns.conn != C_DISCONNECTING && ns.conn <= C_TEAR_DOWN)
 		ns.conn = os.conn;
 
 	/* After C_DISCONNECTING only C_STANDALONE may follow */