rte_mempool_calc_mem_size_helper() was introduced to avoid
code duplication and used in deprecated rte_mempool_mem_size() and
rte_mempool_op_calc_mem_size_default(). Now the first one is removed
and it is better to fold the helper into the second one to make it
more readable.
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>