app/test: fix wraparound of mempool performance rate
app/test: fix wraparound of mempool performance rate

Recent CPU's can easily wrap around a 32-bit unsigned int in
the mempool perf test. Increase to a 64-bit uint.

Signed-off-by: David Hunt <763be7c8e0ad91e03160e7aed76d5cd41534e2f9@intel.com>
Acked-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk