[IPV6] MCAST: Fix joining all-node multicast group on device initialization.

Join all-node multicast group after assignment of dev->ip6_ptr
because it must be assigned when ipv6_dev_mc_inc() is called.
This fixes Bug#7817, reported by <gernoth@informatik.uni-erlangen.de>.

Closes: 7817
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>