net: ipv6: remove 'next' member from inet6_dev

The next pointer within the inet6_dev structure seems not to be used
anywhere. So just remove it. Tested with allmodconfig on x86_64.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>