netfilter: x_tables: add counters allocation wrapper

commit c84ca954 upstream.

allows to have size checks in a single spot.
This is supposed to reduce oom situations when fuzz-testing xtables.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>