ct: add average bytes per packet counter support

ct: add average bytes per packet counter support

Similar to connbytes extension in iptables, now you can use it to match
average bytes per packet a connection has transferred so far.

For example, match avgpkt in "BOTH" diretion:
  # nft add rule x y ct avgpkt \> 100

Match avgpkt in reply direction:
  # nft add rule x y ct reply avgpkt \< 900

Or match avgpkt in original direction:
  # nft add rule x y ct original avgpkt \> 200

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>