hash: fix bucket size usage

Multiwriter insert function was using a fixed value for
the bucket size, instead of using the
RTE_HASH_BUCKET_ENTRIES macro, which value was changed
recently (making it inconsistent in this case).

Fixes: be856325cba3 ("hash: add scalable multi-writer insertion with Intel TSX")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>