netfilter: nft_flow_offload: clear tcp MAXACK flag before moving to slowpath

This state reset is racy, no locks are held here.

Since commit
8437a620 ("netfilter: nft_flow_offload: set liberal tracking mode for tcp"),
the window checks are disabled for normal data packets, but MAXACK flag
is checked when validating TCP resets.

Clear the flag so tcp reset validation checks are ignored.

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