mptcp: pm: re-using ID of unused removed subflows

commit edd8b5d868a4d459f3065493001e293901af758d upstream.

If no subflow is attached to the 'subflow' endpoint that is being
removed, the addr ID will not be marked as available again.

Mark the linked ID as available when removing the 'subflow' endpoint if
no subflow is attached to it.

While at it, the local_addr_used counter is decremented if the ID was
marked as being used to reflect the reality, but also to allow adding
new endpoints after that.

Fixes: b6c08380

 ("mptcp: remove addr and subflow in PM netlink")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240819-net-mptcp-pm-reusing-id-v1-3-38035d40de5b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>