app/testpmd: fix burst percentage calculation

app/testpmd: fix burst percentage calculation

The burst % calculation can over flow due to multiplication.
Fix the multiplication and increase the size of variables to
64b.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Phil Yang <phil.yang@arm.com>
Tested-by: Ali Alnubani <alialnu@mellanox.com>