netfilter: nft_limit: move stateful fields out of expression data

[ Upstream commit 3b9e2ea6 ]

In preparation for the rule blob representation.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 91a139ce ("netfilter: nft_limit: do not ignore unsupported flags")
Signed-off-by: Sasha Levin <sashal@kernel.org>