skbuff: Fix build with SKB extensions disabled

We will fail to build with CONFIG_SKB_EXTENSIONS disabled after
8550ff8d ("skbuff: Release nfct refcount on napi stolen or re-used
skbs") since there is an unconditionally use of skb_ext_find() without
an appropriate stub. Simply build the code conditionally and properly
guard against both COFNIG_SKB_EXTENSIONS as well as
CONFIG_NET_TC_SKB_EXT being disabled.

Fixes: Fixes: 8550ff8d ("skbuff: Release nfct refcount on napi stolen or re-used skbs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -6010,7 +6010,7 @@ static void gro_list_prepare(const struct list_head *head,
 				       maclen);
 
 		diffs |= skb_get_nfct(p) ^ skb_get_nfct(skb);
-
+#if IS_ENABLED(CONFIG_SKB_EXTENSIONS) && IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
 		if (!diffs) {
 			struct tc_skb_ext *skb_ext = skb_ext_find(skb, TC_SKB_EXT);
 			struct tc_skb_ext *p_ext = skb_ext_find(p, TC_SKB_EXT);
@@ -6019,6 +6019,7 @@ static void gro_list_prepare(const struct list_head *head,
 			if (!diffs && unlikely(skb_ext))
 				diffs |= p_ext->chain ^ skb_ext->chain;
 		}
+#endif
 
 		NAPI_GRO_CB(p)->same_flow = !diffs;
 	}