netfilter: ipset: Add skbinfo extension kernel support in the ipset core.

Skbinfo extension provides mapping of metainformation with lookup in the ipset tables.
This patch defines the flags, the constants, the functions and the structures
for the data type independent support of the extension.
Note the firewall mark stores in the kernel structures as two 32bit values,
but transfered through netlink as one 64bit value.

Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>