cfq-iosched: don't delay async queue if it hasn't dispatched at all

cfq-iosched: don't delay async queue if it hasn't dispatched at all

We cannot delay for the first dispatch of the async queue if it
hasn't dispatched at all, since that could present a local user
DoS attack vector using an app that just did slow timed sync reads
while filling memory.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>