app/test: decrease size of requested mempool

app/test: decrease size of requested mempool

In test application, the default size of allocated mempool is calculated
as following:

  (RTE_MAX_LCORE * (RTE_MEMPOOL_CACHE_MAX_SIZE + max_kept_objects)) - 1

The objective is to ensure that all cores can fill their cache and keep
'max_kept_objects' at the same time. As RTE_MAX_LCORE is 128 and
RTE_MEMPOOL_CACHE_MAX_SIZE is 512 in the default configuration, it can
produce very large mempools (170 MB).

We can replace the number of core by a dynamic value, which drastically
reduces the amount of memory needed for this test (5 MB with 4 cores).

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>