netlink: add support to set meta keys

netlink: add support to set meta keys

Arturo Borrero added kernel support to set meta keys in
http://patchwork.ozlabs.org/patch/305281/ and the corresponding
library support in http://patchwork.ozlabs.org/patch/305283/.

This patch enhances nft to use this new kernel feature. The
following example shows how to set the packet mark.

% nft add rule ip filter input meta mark set 22
% nft list table filter
table ip filter {
        chain input {
                 type filter hook input priority 0;
                 meta mark set 0x00000016
        }
}

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>