ipv6: fix bad free of addrconf_init_net

[ Upstream commit a79ca223e029aa4f09abb337accf1812c900a800 ]

Signed-off-by: Hong Zhiguo <honkiko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>