netfilter: nft_quota: dump consumed quota

Add a new attribute NFTA_QUOTA_CONSUMED that displays the amount of
quota that has been already consumed. This allows us to restore the
internal state of the quota object between reboots as well as to monitor
how wasted it is.

This patch changes the logic to account for the consumed bytes, instead
of the bytes that remain to be consumed.

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