l2tplac: fast-forward session that triggered tunnel creation

Previously, when we got a call to be forwarded to another LNS and we didn't
have a tunnel to it yet, we would wait for a PAP/CHAP re-send to trigger the
creation of a session in the fresh tunnel. We do not have to wait, we can
trigger that on SCCCN ack.

l2tpns.h

@@ -520,6 +520,8 @@ typedef struct
 	int32_t scccn;		// seq number of last sccn waiting for an ack (-1 if none)
 	controlt *controlr;	// queue of OoO-received messages
 	int l2tp_fd;		// kernel acceleration UDP socket
+	sessionidt initialsession;	// initial session that triggered creating the tunnel to another LNS
+	char initialuser[MAXUSER];	// user of initial session
 }
 tunnellocalt;