[IPV6] ADDRCONF: Fix array size for sysctls.

We have been using __NET_IPV6_MAX for adjusting the size of array
for sysctl table, but it does not work any longer because of the
deprecation of NET_IPV6_xxx constants.  Let's use DEVCONF_MAX
instead.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>