netfilter: nft_limit: rename stateful structure

[ Upstream commit 369b6cb5 ]

From struct nft_limit to nft_limit_priv.

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>