tcp: factor out __tcp_close() helper

[ Upstream commit 77c3c95637526f1e4330cc9a4b2065f668c2c4fe ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>