selftests: mptcp: add the id argument for set_flags

This patch added the id argument for setting the address flags in
pm_nl_ctl.

Usage:

    pm_nl_ctl set id 1 flags backup

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>