[MACVLAN]: Update Kconfig to refer to iproute

Since the macvlan release I had at least 5 users asking how to configure
it since the old userspace tool doesn't work with the version in the
kernel. Add a pointer to the Kconfig help.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>