Reduce netperf runtime from netperf start overhead

Fristly according strategy, the largest timeout_netperf_start
should be int(l) * 0.5, if netperf thread start at once, it
will still need int(l) * 0.5 time waste, that's overhead does
not needed.

Signed-off-by: Wenli Quan <wquan@redhat.com>