netfilter: nf_tables: improve nft payload fast eval

There's an off-by-one issue in nft_payload_fast_eval, skb_tail_pointer
and ptr + priv->len all point to the last valid address plus 1. So if
they are equal, we can still fetch the valid data. It's unnecessary to
fall back to nft_payload_eval.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_core.c

@@ -98,7 +98,7 @@ static bool nft_payload_fast_eval(const struct nft_expr *expr,
 
 	ptr += priv->offset;
 
-	if (unlikely(ptr + priv->len >= skb_tail_pointer(skb)))
+	if (unlikely(ptr + priv->len > skb_tail_pointer(skb)))
 		return false;
 
 	*dest = 0;