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>

tools/testing/selftests/net/mptcp/pm_nl_ctl.c

@@ -31,7 +31,7 @@
 
 static void syntax(char *argv[])
 {
-	fprintf(stderr, "%s add|get|set|del|flush|dump|accept [<args>]\n", argv[0]);
+	fprintf(stderr, "%s add|ann|rem|csf|dsf|get|set|del|flush|dump|events|listen|accept [<args>]\n", argv[0]);
 	fprintf(stderr, "\tadd [flags signal|subflow|backup|fullmesh] [id <nr>] [dev <name>] <ip>\n");
 	fprintf(stderr, "\tann <local-ip> id <local-id> token <token> [port <local-port>] [dev <name>]\n");
 	fprintf(stderr, "\trem id <local-id> token <token>\n");