Some code cleanup related to netif index handling

Made the macro "netif_index_to_num" private, if someone needs it externally, please complain.

src/core/netif.c

@@ -104,6 +104,7 @@
 struct netif *netif_list;
 struct netif *netif_default;
 
+#define netif_index_to_num(index)   ((index) - 1)
 static u8_t netif_num;
 
 #if LWIP_NUM_NETIF_CLIENT_DATA > 0
@@ -1309,7 +1310,7 @@ netif_name_to_index(const char *name)
 {
   struct netif *netif = netif_find(name);
   if (netif != NULL) {
-    return netif_num_to_index(netif);
+    return netif_get_index(netif);
   }
   /* No name found, return invalid index */
   return 0;
@@ -1345,3 +1346,25 @@ netif_index_to_name(u8_t idx, char *name)
   }
   return NULL;
 }
+
+/**
+* @ingroup netif
+* Return the interface for the netif index
+*
+* @param index index of netif to find
+*/
+struct netif*
+netif_get_by_index(u8_t index)
+{
+  struct netif* netif;
+
+  if (index != NETIF_NO_INDEX) {
+    for (netif = netif_list; netif != NULL; netif = netif->next) {
+      if (index == netif_get_index(netif)) {
+        return netif; /* found! */
+      }
+    }
+  }
+
+  return NULL;   
+}

src/core/raw.c

@@ -327,15 +327,11 @@ raw_sendto(struct raw_pcb *pcb, struct pbuf *p, const ip_addr_t *ipaddr)
 
 #if LWIP_MULTICAST_TX_OPTIONS
   netif = NULL;
-  if (ip_addr_ismulticast(ipaddr) && (pcb->mcast_ifindex != NETIF_NO_INDEX)) {
+  if (ip_addr_ismulticast(ipaddr)) {
     /* For multicast-destined packets, use the user-provided interface index to
      * determine the outgoing interface, if an interface index is set and a
      * matching netif can be found. Otherwise, fall back to regular routing. */
-    for (netif = netif_list; netif != NULL; netif = netif->next) {
-      if (pcb->mcast_ifindex == netif_num_to_index(netif)) {
-        break; /* found! */
-      }
-    }
+    netif = netif_get_by_index(pcb->mcast_ifindex);
   }
 
   if (netif == NULL)

src/core/udp.c

@@ -540,11 +540,7 @@ udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *dst_ip,
      * list, but by doing so we skip a route lookup. If the interface index has
      * gone stale, we fall through and do the regular route lookup after all. */
     if (pcb->mcast_ifindex != NETIF_NO_INDEX) {
-      for (netif = netif_list; netif != NULL; netif = netif->next) {
-        if (pcb->mcast_ifindex == netif_num_to_index(netif)) {
-          break; /* found! */
-        }
-      }
+      netif = netif_get_by_index(pcb->mcast_ifindex);
     }
 #if LWIP_IPV4
     else