Case 1: if net.ipv6.conf.IFACE.forwarding has been set to 1,
we multiply join routers' multicast/anycast addresses 
by setting net.ipv6.conf.all.forwarding to 1.

Noticed by Andre Tomt <andre@tomt.net>.

Case 2: if we change net.ipv6.conf.FOO.forwarding from 1 to 2,
we multiply join routers' multicast/anycast addresses.
Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>