netfilter: nft_flow_offload: update tcp state flags under lock

The conntrack entry is already public, there is a small chance that another
CPU is handling a packet in reply direction and racing with the tcp state
update.

Move this under ct spinlock.

This is done once, when ct is about to be offloaded, so this should
not result in a noticeable performance hit.

Fixes: 8437a620 ("netfilter: nft_flow_offload: set liberal tracking mode for tcp")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>