mptcp: pm: send ACK on non-stale subflows

If the subflow is considered as "staled", it is better to avoid it to
send an ACK carrying an ADD_ADDR or RM_ADDR. Another subflow, if any,
will then be selected.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241021-net-next-mptcp-misc-6-13-v1-1-1ef02746504a@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>