examples/l2fwd-keepalive: fix integer overflow
examples/l2fwd-keepalive: fix integer overflow

Fix Coverity warning with potential 32-bit integer multiplication overflow
where final result is expected to be 64-bit.

>>> CID 120144 (#1 of 1): Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)

Fixes: e64833f2273a ("examples/l2fwd-keepalive: add sample application")

Signed-off-by: Remy Horton <34c343e92f0e27f7a8e26a1d8d2146ae7c7dd1a3@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

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