netfilter: flowtable: add hash offset field to tuple

Add a placeholder field to calculate hash tuple offset. Similar to
2c407aca ("netfilter: conntrack: avoid gcc-10 zero-length-bounds
warning").
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_flow_table.h

@@ -107,6 +107,10 @@ struct flow_offload_tuple {
 
 	u8				l3proto;
 	u8				l4proto;
+
+	/* All members above are keys for lookups, see flow_offload_hash(). */
+	struct { }			__hash;
+
 	u8				dir;
 
 	u16				mtu;

net/netfilter/nf_flow_table_core.c

@@ -191,14 +191,14 @@ static u32 flow_offload_hash(const void *data, u32 len, u32 seed)
 {
 	const struct flow_offload_tuple *tuple = data;
 
-	return jhash(tuple, offsetof(struct flow_offload_tuple, dir), seed);
+	return jhash(tuple, offsetof(struct flow_offload_tuple, __hash), seed);
 }
 
 static u32 flow_offload_hash_obj(const void *data, u32 len, u32 seed)
 {
 	const struct flow_offload_tuple_rhash *tuplehash = data;
 
-	return jhash(&tuplehash->tuple, offsetof(struct flow_offload_tuple, dir), seed);
+	return jhash(&tuplehash->tuple, offsetof(struct flow_offload_tuple, __hash), seed);
 }
 
 static int flow_offload_hash_cmp(struct rhashtable_compare_arg *arg,
@@ -207,7 +207,7 @@ static int flow_offload_hash_cmp(struct rhashtable_compare_arg *arg,
 	const struct flow_offload_tuple *tuple = arg->key;
 	const struct flow_offload_tuple_rhash *x = ptr;
 
-	if (memcmp(&x->tuple, tuple, offsetof(struct flow_offload_tuple, dir)))
+	if (memcmp(&x->tuple, tuple, offsetof(struct flow_offload_tuple, __hash)))
 		return 1;
 
 	return 0;