mptcp: refactor passive socket initialization

After commit 30e51b92 ("mptcp: fix unreleased socket in accept queue")
unaccepted msk sockets go throu complete shutdown, we don't need anymore
to delay inserting the first subflow into the subflow lists.

The reference counting deserve some extra care, as __mptcp_close() is
unaware of the request socket linkage to the first subflow.

Please note that this is more a refactoring than a fix but because this
modification is needed to include other corrections, see the following
commits. Then a Fixes tag has been added here to help the stable team.

Fixes: 30e51b92 ("mptcp: fix unreleased socket in accept queue")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Tested-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>