block: drbd: remove a stray unlock in __drbd_send_protocol()

There are two callers of this function and they both unlock the mutex so
this ends up being a double unlock.

Fixes: 44ed167d ("drbd: rcu_read_lock() and rcu_dereference() for tconn->net_conf")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_main.c

@@ -786,7 +786,6 @@ int __drbd_send_protocol(struct drbd_connection *connection, enum drbd_packet cm
 
 	if (nc->tentative && connection->agreed_pro_version < 92) {
 		rcu_read_unlock();
-		mutex_unlock(&sock->mutex);
 		drbd_err(connection, "--dry-run is not supported by peer");
 		return -EOPNOTSUPP;
 	}