selftests: mptcp: add token for dump_addr

The command dump_addr() of pm_nl_ctl can be used like this in in-kernel PM:

        pm_nl_ctl dump

This patch adds token argument for it to support userspace PM:

        pm_nl_ctl dump token $token

If 'token $token' is passed to dump_addr(), copy it into the kernel
netlink.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>