generic.netperf: Reduce netperf runtime from netperf start overhead

The largest timeout_netperf_start should be int(l) * 0.5,
if netperf threads start all at once, it will still need
int(l) * 0.5 time waste. Therefore, the netperf_start_timeout
overhead is not needed.

Signed-off-by: Wenli Quan <wquan@redhat.com>