gro: move the tc_ext comparison to a helper

The double ifdefs (one for the variable declaration and
one around the code) are quite aesthetically displeasing.
Factor this code out into a helper for easier wrapping.

This will become even more ugly when another skb ext
comparison is added in the future.

The resulting machine code looks the same, the compiler
seems to try to use %rax more and some blocks more around
but I haven't spotted minor differences.
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/gro.c

@@ -304,6 +304,24 @@ void napi_gro_flush(struct napi_struct *napi, bool flush_old)
 }
 EXPORT_SYMBOL(napi_gro_flush);
 
+static unsigned long gro_list_prepare_tc_ext(const struct sk_buff *skb,
+					     const struct sk_buff *p,
+					     unsigned long diffs)
+{
+#if IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
+	struct tc_skb_ext *skb_ext;
+	struct tc_skb_ext *p_ext;
+
+	skb_ext = skb_ext_find(skb, TC_SKB_EXT);
+	p_ext = skb_ext_find(p, TC_SKB_EXT);
+
+	diffs |= (!!p_ext) ^ (!!skb_ext);
+	if (!diffs && unlikely(skb_ext))
+		diffs |= p_ext->chain ^ skb_ext->chain;
+#endif
+	return diffs;
+}
+
 static void gro_list_prepare(const struct list_head *head,
 			     const struct sk_buff *skb)
 {
@@ -338,23 +356,11 @@ static void gro_list_prepare(const struct list_head *head,
 		 * avoid trying too hard to skip each of them individually
 		 */
 		if (!diffs && unlikely(skb->slow_gro | p->slow_gro)) {
-#if IS_ENABLED(CONFIG_SKB_EXTENSIONS) && IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
-			struct tc_skb_ext *skb_ext;
-			struct tc_skb_ext *p_ext;
-#endif
-
 			diffs |= p->sk != skb->sk;
 			diffs |= skb_metadata_dst_cmp(p, skb);
 			diffs |= skb_get_nfct(p) ^ skb_get_nfct(skb);
 
-#if IS_ENABLED(CONFIG_SKB_EXTENSIONS) && IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
-			skb_ext = skb_ext_find(skb, TC_SKB_EXT);
-			p_ext = skb_ext_find(p, TC_SKB_EXT);
-
-			diffs |= (!!p_ext) ^ (!!skb_ext);
-			if (!diffs && unlikely(skb_ext))
-				diffs |= p_ext->chain ^ skb_ext->chain;
-#endif
+			diffs |= gro_list_prepare_tc_ext(skb, p, diffs);
 		}
 
 		NAPI_GRO_CB(p)->same_flow = !diffs;