MT#55283 unify skb_pull to transport

Use the existing offsets to network/transport header to determine how
much needs to be pulled back. Then we can combine these code snippets
into one.

No functional change.

Change-Id: I34eeb707da9baad0891d2fbb2d8d9fc95e027204

kernel-module/nft_rtpengine.c

@@ -7301,12 +7301,14 @@ static int rtpengine4(struct sk_buff *oskb, struct net *net, struct rtpengine_ta
 		return NFT_CONTINUE;
 
 	skb_gso_reset(skb);
-	skb_reset_network_header(skb);
 	ih = ip_hdr(skb);
-	skb_pull(skb, (ih->ihl << 2));
+
 	if (ih->protocol != IPPROTO_UDP)
 		goto out;
 
+	// pull to transport (UDP) header
+	skb_pull(skb, skb->transport_header - skb->network_header);
+
 	memset(&src, 0, sizeof(src));
 	memset(&dst, 0, sizeof(dst));
 	src.family = AF_INET;
@@ -7348,13 +7350,14 @@ static int rtpengine6(struct sk_buff *oskb, struct net *net, struct rtpengine_ta
 		return NFT_CONTINUE;
 
 	skb_gso_reset(skb);
-	skb_reset_network_header(skb);
 	ih = ipv6_hdr(skb);
 
-	skb_pull(skb, sizeof(*ih));
 	if (ih->nexthdr != IPPROTO_UDP)
 		goto out;
 
+	// pull to transport (UDP) header
+	skb_pull(skb, skb->transport_header - skb->network_header);
+
 	memset(&src, 0, sizeof(src));
 	memset(&dst, 0, sizeof(dst));
 	src.family = AF_INET6;