tcp: factor out __tcp_close() helper

stable inclusion
from stable-v4.19.284
commit e62806034c409b87e8fa00b829c059ba8face62c
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7J5UF


CVE: NA

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[ Upstream commit 77c3c956 ]

unlocked version of protocol level close, will be used by
MPTCP to allow decouple orphaning and subflow level close.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: e14cadfd ("tcp: add annotations around sk->sk_shutdown accesses")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>