mptcp: pm: use pm variable instead of msk->pm

mptcp: pm: use pm variable instead of msk->pm

The variable "pm" has been defined in mptcp_pm_fully_established()
and mptcp_pm_data_reset() as "msk->pm", so use "pm" directly instead
of using "msk->pm".

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>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250313-net-next-mptcp-pm-ops-intro-v1-3-f4e4a88efc50@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>