This patch adds the quota expression. This new stateful expression
integrate easily into the dynset expression to build 'hashquota' flow
tables.

Arguably, we could use instead "counter bytes > 1000" instead, but this
approach has several problems:

1) We only support for one single stateful expression in dynamic set
   definitions, and the expression above is a composite of two
   expressions: get counter + comparison.

2) We would need to restore the packed counter representation (that we
   used to have) based on seqlock to synchronize this, since per-cpu is
   not suitable for this.

So instead of bloating the counter expression back with the seqlock
representation and extending the existing set infrastructure to make it
more complex for the composite described above, let's follow the more
simple approach of adding a quota expression that we can plug into our
existing infrastructure.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>