netfilter: nft_ct: make byte/packet expr more friendly

If we want to use ct packets expr, and add a rule like follows:
  # nft add rule filter input ct packets gt 1 counter

We will find that no packets will hit it, because
nf_conntrack_acct is disabled by default. So It will
not work until we enable it manually via
"echo 1 > /proc/sys/net/netfilter/nf_conntrack_acct".

This is not friendly, so like xt_connbytes do, if the user
want to use ct byte/packet expr, enable nf_conntrack_acct
automatically.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>