netfilter: ebt_among: obtain match size through different means

The function signatures will be changed to match those of Xtables, and
the datalen argument will be gone. ebt_among unfortunately relies on
it, so we need to obtain it somehow.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>