netfilter: ipset: Add element count to hash headers

It would be useful for userspace to query the size of an ipset hash,
however, this data is not exposed to userspace outside of counting the
number of member entries.  This patch uses the attribute
IPSET_ATTR_ELEMENTS to indicate the size in the the header that is
exported to userspace.  This field is then printed by the userspace
tool for hashes.

Signed-off-by: Eric B Munson <emunson@akamai.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Josh Hunt <johunt@akamai.com>
Cc: netfilter-devel@vger.kernel.org
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>