src: Add cgroup support in meta expresion

src: Add cgroup support in meta expresion

The new attribute of meta is "cgroup".

Example of use in nft:

 # nft add rule ip test output meta cgroup != 0x100001 counter drop

Moreover, this adds tests to the meta.t test file.

The kernel support is addedin the commit:
ce67417 ("netfilter: nft_meta: add cgroup support")

The libnftnl  support is add in the commit:
1d4a480 ("expr: meta: Add cgroup support")

More information about the steps to use cgroup:
https://www.kernel.org/doc/Documentation/cgroups/net_cls.txt

More info about cgroup in iptables:
http://git.kernel.org/cgit/linux/kernel/git/pablo/nftables.git/commit/net/netfilter/xt_cgroup.c?id=82a37132f300ea53bdcd812917af5a6329ec80c3

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>