mptcp: sysctl: map pm_type to path_manager

mptcp: sysctl: map pm_type to path_manager

This patch adds a new proc_handler "proc_pm_type" for "pm_type" to
map old path manager sysctl "pm_type" to the newly added "path_manager".

	path_manager		   pm_type

	MPTCP_PM_TYPE_KERNEL    -> "kernel"
	MPTCP_PM_TYPE_USERSPACE -> "userspace"

It is important to add this to keep a compatibility with the now
deprecated pm_type sysctl knob.

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-v1-10-f4e4a88efc50@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>