drbd: fix broken state change after split-brain attach while connected

Situation:
we have diverging data sets, i.e. we had a split brain somewhen,
but currently are connected, one node diskless.

Then we try to attach that disk, figure it is consistent,
but has a diverging data set, we refuse to attach.

This led to strange state changes:
22:18:35 bb drbd1: peer( Unknown -> Primary ) conn( WFReportParams -> Connected) pdsk( DUnknown -> UpToDate )
22:19:30 bb drbd1: disk( Diskless -> Attaching )
22:19:30 bb drbd1: disk( Attaching -> Negotiating )
22:19:30 bb drbd1: drbd_sync_handshake:
22:19:30 bb drbd1: self 97BF25798B9D5222:F33D1F62ADE698DD:4269796F9D027C83:AC45D8B5C3C1BF93 bits:19449 flags:0
22:19:30 bb drbd1: peer 280DFB6E125465D3:F33D1F62ADE698DC:4269796F9D027C82:AC45D8B5C3C1BF93 bits:2575806 flags:0
22:19:30 bb drbd1: uuid_compare()=100 by rule 90
22:19:30 bb drbd1: Split-Brain detected, dropping connection!
22:19:30 bb drbd1: disk( Negotiating -> Diskless )

while the other side says:
22:19:30 aa drbd1: Split-Brain detected, dropping connection!
22:19:30 aa drbd1: Disk attach process on the peer node was aborted.
22:19:30 aa drbd1: conn( Connected -> TOO_LARGE ) pdsk( Diskless -> Consistent )

This should be fixed now.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_receiver.c

@@ -2513,6 +2513,10 @@ static enum drbd_conns drbd_sync_handshake(struct drbd_conf *mdev, enum drbd_rol
 	}
 
 	if (hg == -100) {
+		/* FIXME this log message is not correct if we end up here
+		 * after an attempted attach on a diskless node.
+		 * We just refuse to attach -- well, we drop the "connection"
+		 * to that disk, in a way... */
 		dev_alert(DEV, "Split-Brain detected, dropping connection!\n");
 		drbd_khelper(mdev, "split-brain");
 		return C_MASK;
@@ -3134,12 +3138,13 @@ static int receive_state(struct drbd_conf *mdev, struct p_header *h)
 
 		put_ldev(mdev);
 		if (nconn == C_MASK) {
+			nconn = C_CONNECTED;
 			if (mdev->state.disk == D_NEGOTIATING) {
 				drbd_force_state(mdev, NS(disk, D_DISKLESS));
-				nconn = C_CONNECTED;
 			} else if (peer_state.disk == D_NEGOTIATING) {
 				dev_err(DEV, "Disk attach process on the peer node was aborted.\n");
 				peer_state.disk = D_DISKLESS;
+				real_peer_disk = D_DISKLESS;
 			} else {
 				if (test_and_clear_bit(CONN_DRY_RUN, &mdev->flags))
 					return FALSE;