mptcp: pm: split netlink and in-kernel init

mptcp: pm: split netlink and in-kernel init

The registration of mptcp_genl_family is useful for both the in-kernel
and the userspace PM. It should then be done in pm_netlink.c.

On the other hand, the registration of the in-kernel pernet subsystem is
specific to the in-kernel PM, and should stay there in pm_kernel.c.

Reviewed-by: Mat Martineau <martineau@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-v1-1-f4e4a88efc50@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>