drbd: Always use the same protocol version for the same peer

There is no need to send protocol 80 headers to peers that understand
protocol 95 headers.  Make sure that we don't send protocol 95 headers
until we have agreed upon a protocol version with our peer, though.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_main.c

@@ -706,7 +706,7 @@ static void prepare_header95(struct p_header95 *h, enum drbd_packet cmd, int siz
 static void _prepare_header(struct drbd_tconn *tconn, int vnr, struct p_header *h,
 			    enum drbd_packet cmd, int size)
 {
-	if (tconn->agreed_pro_version >= 100 || size > DRBD_MAX_SIZE_H80_PACKET)
+	if (tconn->agreed_pro_version >= 95)
 		prepare_header95(&h->h95, cmd, size);
 	else
 		prepare_header80(&h->h80, cmd, size);

drivers/block/drbd/drbd_receiver.c

@@ -819,9 +819,9 @@ static int drbd_connect(struct drbd_tconn *tconn)
 		return -2;
 
 	clear_bit(DISCARD_CONCURRENT, &tconn->flags);
-	tconn->agreed_pro_version = 99;
-	/* agreed_pro_version must be smaller than 100 so we send the old
-	   header (h80) in the first packet and in the handshake packet. */
+
+	/* Assume that the peer only understands protocol 80 until we know better.  */
+	tconn->agreed_pro_version = 80;
 
 	sock  = NULL;
 	msock = NULL;