ip: add nlmon as a device type to help message

Though nlmon device can be added, it was not listed
in the output of "ip link help".
Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>

ip/iplink.c

@@ -87,7 +87,7 @@ void iplink_usage(void)
 		fprintf(stderr, "\n");
 		fprintf(stderr, "TYPE := { vlan | veth | vcan | dummy | ifb | macvlan | macvtap |\n");
 		fprintf(stderr, "          bridge | bond | ipoib | ip6tnl | ipip | sit | vxlan |\n");
-		fprintf(stderr, "          gre | gretap | ip6gre | ip6gretap | vti }\n");
+		fprintf(stderr, "          gre | gretap | ip6gre | ip6gretap | vti | nlmon }\n");
 	}
 	exit(-1);
 }