mptcp: sysctl: map path_manager to pm_type

This patch maps the newly added path manager sysctl "path_manager"
to the old one "pm_type".

	path_manager   pm_type

	"kernel"    -> MPTCP_PM_TYPE_KERNEL
	"userspace" -> MPTCP_PM_TYPE_USERSPACE
	others      -> __MPTCP_PM_TYPE_NR

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-v1-9-f4e4a88efc50@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>