sch_red: fix red_calc_qavg_from_idle_time

Since commit a4a710c4

 (pkt_sched: Change PSCHED_SHIFT from 10 to
6) it seems RED/GRED are broken.

red_calc_qavg_from_idle_time() computes a delay in us units, but this
delay is now 16 times bigger than real delay, so the final qavg result
smaller than expected.

Use standard kernel time services since there is no need to obfuscate
them.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>