netfilter: nft_flow_offload: don't offload when sequence numbers need adjustment

We can't deal with tcp sequence number rewrite in flow_offload.
While at it, simplify helper check, we only need to know if the extension
is present, we don't need the helper data.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>