test/ring: fix statistics in bulk enq/dequeue

test/ring: fix statistics in bulk enq/dequeue

In size 32 bulk ring enq/dequeue performance test, the "Total count"
statistics is incorrect. For example, running the test on lcore 25 and
lcore 26, the output is as follows:

The test command:
$sudo ./arm64-armv8a-linuxapp-gcc/app/test -l 25-26
RTE>>ring_perf_autotest

Bulk enq/dequeue count on size 32
Core [25] count = 288268
Core [26] count = 288281
Total count (size: 32): 1066323

Fixed it by reset the counter at the beginning of each loop. The
revised output is as follows:

Bulk enq/dequeue count on size 32
Core [25] count = 285643
Core [26] count = 285688
Total count (size: 32): 571331

Fixes: 759cf9b5632c ("test/ring: enhance mp/mc coverage")
Cc: stable@dpdk.org
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>