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: Chris Wright <chrisw@sous-sol.org>