move private struct dns_server_interface to c file

use ip_addr function to initialize dns_add_interface_server

Author: JojoS62

connectivity/lwipstack/lwip/src/core/lwip_dns.c

@@ -184,6 +184,14 @@ static u16_t dns_txid;
 #define LWIP_DNS_ISMDNS_ARG(x)
 #endif
 
+/** dns_server_interface structue
+    used locally for multihoming systems. */
+struct dns_server_interface {
+    char interface_name [INTERFACE_NAME_MAX_SIZE];
+    ip_addr_t dns_servers[DNS_MAX_SERVERS];
+    struct dns_server_interface *next;
+};
+
 /** DNS query message structure.
     No packing needed: only used locally on the stack. */
 struct dns_query {
@@ -431,8 +439,11 @@ dns_add_interface_server(u8_t numdns, const char *interface_name, const ip_addr_
     }
   }
   // add new dns server to the list tail
-  new_interface_server = mem_calloc(1, sizeof(struct dns_server_interface));
+  new_interface_server = mem_malloc(sizeof(struct dns_server_interface));
   snprintf(new_interface_server->interface_name, INTERFACE_NAME_MAX_SIZE, "%s",interface_name);
+  for (u32_t i=0; i<DNS_MAX_SERVERS; i++) {
+    ip_addr_set_zero(&new_interface_server->dns_servers[i]);  
+  }
   new_interface_server->dns_servers[numdns] = (*dnsserver);
   new_interface_server->next = NULL;
 

connectivity/lwipstack/lwip/src/include/lwip/dns.h

@@ -50,12 +50,6 @@
 extern "C" {
 #endif
 
-struct dns_server_interface {
-    char interface_name [INTERFACE_NAME_MAX_SIZE];
-    ip_addr_t dns_servers[DNS_MAX_SERVERS];
-    struct dns_server_interface *next;
-};
-
 /** DNS timer period */
 #define DNS_TMR_INTERVAL          1000