netfilter: nftables: add loop check helper function

[ Upstream commit 6387aa6e ]

This patch adds nft_check_loops() to reuse it in the new catch-all
element codebase.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: b29be0ca ("netfilter: nft_immediate: drop chain reference counter on error")
Signed-off-by: Sasha Levin <sashal@kernel.org>