[IPV6] MROUTE: Do not call ipv6_find_idev() directly.

Since NETDEV_REGISTER notifier chain is responsible for creating
inet6_dev{}, we do not need to call ipv6_find_idev() directly here.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>