netfilter: ipset: allocate the proper memory for the generic hash structure

netfilter: ipset: allocate the proper memory for the generic hash structure

Because a single create function is emitted for every hash type,
from the IPv4 and IPv6 generic hash structure definitions the last
one, i.e. the IPv6 was in effect for IPv4 too. Use the proper size
when allocating the structure. Comment properly that because create()
refers to elements of the generic hash structure, all referred ones
must come before the IPv4/IPv6 dependent 'next' member.

Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>