rtnetlink: advertise allmulti counter

Like what was done with IFLA_PROMISCUITY, add IFLA_ALLMULTI to advertise
the allmulti counter.
The flag IFF_ALLMULTI is advertised only if it was directly set by a
userland app.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>