app/flow-perf: fix IPv6 source address increment

app/flow-perf: fix IPv6 source address increment

Currently the memset() will not set a correct src ip that represent
the incremental value of the counter.

This commit will fix this and each flow will have correct IPv6.src
that it's incremental from previous flow and equal to the decimal
values.

Fixes: bf3688f1e816 ("app/flow-perf: add insertion rate calculation")
Cc: stable@dpdk.org
Signed-off-by: Wisam Jaddo <wisamm@nvidia.com>
Acked-by: Alexander Kozyrev <akozyrev@nvidia.com>