net/route: fix return value of net_writeroute_ipv6

Return ntotal (total bytes written) instead of ret (result of last
write call). This matches the expected semantics and is consistent
with net_writeroute_ipv4().

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/route/net_fileroute.c

@@ -475,7 +475,7 @@ ssize_t net_writeroute_ipv6(FAR struct file *filep,
   while (ntotal < sizeof(struct net_route_ipv6_s));
 
   net_unlockroute_ipv6();
-  return ret;
+  return ntotal;
 }
 #endif