netfilter: conntrack: place confirm-bit setting in a helper

mainline inclusion
from mainline-v5.6-rc3
commit b1b32552
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I63OS1


CVE: NA

--------------------------------

... so it can be re-used from clash resolution in followup patch.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sun Shouxin <sunshouxin@chinatelecom.cn>
Signed-off-by: Xibo.Wang <wangxb12@chinatelecom.cn>

conflict:
	net/netfilter/nf_conntrack_core.c

Signed-off-by: Lu Wei <luwei32@huawei.com>