selftests: mptcp: update pm_nl_ctl usage header

The usage header of pm_nl_ctl command doesn't match with the context. So
this patch adds the missing userspace PM keywords 'ann', 'rem', 'csf',
'dsf', 'events' and 'listen' in it.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>