IPv6: Fix not join all-router mcast group when forwarding set.

[ Upstream commit d6ddef9e641d1229d4ec841dc75ae703171c3e92 ]

When forwarding was set and a new net device is register,
we need add this device to the all-router mcast group.

Signed-off-by: Li Wei <lw@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>