netfilter: nft_limit: add per-byte limiting

This patch adds a new NFTA_LIMIT_TYPE netlink attribute to indicate the type of
limiting.

Contrary to per-packet limiting, the cost is calculated from the packet path
since this depends on the packet length.

The burst attribute indicates the number of bytes in which the rate can be
exceeded.

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