mptcp: pm: remove '_nl' from mptcp_pm_nl_subflow_chk_stale()

mptcp: pm: remove '_nl' from mptcp_pm_nl_subflow_chk_stale()

Currently, in-kernel PM specific helpers are prefixed with
'mptcp_pm_nl_'. But here 'mptcp_pm_nl_subflow_chk_stale' is not specific
to this PM: it is called from pm.c for both the in-kernel and userspace
PMs.

To avoid confusions, the '_nl' bit has been removed from the name.

No behavioural changes intended.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250307-net-next-mptcp-pm-reorg-v1-6-abef20ada03b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>