PPP, PPPoL2TP, removed unnecessary single pbuf

Removed ppp_singlebuf() in pppol2tp_input(), chained pbuf are perfectly
acceptable for IP data and we are currently supporting them perfectly
for PPPoS. The PPP stack itself (LCP, IPCP et al.) does not support
chained pbuf and is already calling ppp_singlebuf() just before passing
packet to the protocol handler.

Added ppp_singlebuf() in pppol2tp_dispatch_control_packet() because we
do not support chained pbuf in L2TP control packet.

src/netif/ppp/pppol2tp.c

@@ -376,7 +376,6 @@ static void pppol2tp_input(void *arg, struct udp_pcb *pcb, struct pbuf *p, const
   }
 
   /* printf("-----------\nL2TP INPUT, %d\n", p->len); */
-  p = ppp_singlebuf(p);
 
   /* L2TP header */
   if (p->len < sizeof(hflags) + sizeof(tunnel_id) + sizeof(session_id) ) {
@@ -519,6 +518,7 @@ static void pppol2tp_dispatch_control_packet(pppol2tp_pcb *l2tp, u16_t port, str
     return;
   }
 
+  p = ppp_singlebuf(p);
   inp = (u8_t*)p->payload;
   /* Decode AVPs */
   while (p->len > 0) {