Fix default interface ID only being used partially

If user sets the default interface ID for a socket (e.g. using setsockopt
with SOCKET_INTERFACE_SELECT), the default interface should take over
other interface selection mechanisms as a interface is bound to the socket.
This applies for both IPv6 local and global scopes for unicast messages
but not for multicast messages as these are bound to a multicast interface
using SOCKET_IPV6_MULTICAST_IF socket option.

Author: YannCharbon

connectivity/nanostack/sal-stack-nanostack/source/Core/ns_socket.c

@@ -1570,8 +1570,17 @@ struct protocol_interface_info_entry *socket_interface_determine(const socket_t
         }
     }
 
-    /* Try a routing table entry for greater-than-realm scope */
+    /* For greater-than-realm scope, use default interface if a default interface ID */
+    /* has been set (e.g. using setsockopt), else try a routing table entry */
     if (addr_ipv6_scope(buf->dst_sa.address, NULL) > IPV6_SCOPE_REALM_LOCAL) {
+        if (socket_ptr->default_interface_id != -1) {
+            cur_interface = protocol_stack_interface_info_get_by_id(socket_ptr->default_interface_id);
+            if (cur_interface) {
+                return cur_interface;
+            } else {
+                return NULL;
+            }
+        }
         if (ipv6_buffer_route(buf)) {
             return buf->interface;
         }