test/rwlock: amortize the cost of getting time
test/rwlock: amortize the cost of getting time

Instead of getting timestamp per iteration, amortize its
overhead can help to get more precise benchmarking results.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org

Signed-off-by: Joyce Kong <c01c57df07e2f34c67ea70652554926ebc164737@arm.com>
Acked-by: Konstantin Ananyev <cbd5212b59ab42a210d72992d23a3aa17cfd3eaf@intel.com>

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