mptcp: implement mptcp_pm_connection_closed

mptcp: implement mptcp_pm_connection_closed

The MPTCP path manager event handler mptcp_pm_connection_closed
interface has been added in the commit 1b1c7a0ef7f3 ("mptcp: Add path
manager interface") but it was an empty function from then on.

With such name, it sounds good to invoke mptcp_event with the
MPTCP_EVENT_CLOSED event type from it. It also removes a bit of
duplicated code.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241021-net-next-mptcp-misc-6-13-v1-3-1ef02746504a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>