random read cause repeat prefetch same blocks

[sjp00556]

What happened:
random read will cause repeat prefetch same blocks，which will case bandwidth amplification

How to reproduce it (as minimally and precisely as possible):
fio -filename=./f1 -direct=1 -iodepth 1 -thread -rw=randread -ioengine=psync -bs=128K -size=100m -numjobs=1 -group_reporting -name=mytest

the same key may be prefetched repeatedly

[SandyXSD]

Fixed by #4947.