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>