Prefetching appears to have a slightly negative effect on boot time, and
actually makes boot time much worse when the available dm-bufio cache
shrinks. Since we anticipate the dm-bufio cache will shrink (because of
an increasing number of APEX dm-verity targets), disable prefetching
completely.
Boot time w/o this change: 12.085
Boot time with this change: 11.958
Boot time w/o this change with 128kB dm-bufio cache: 22.82
Boot time with this change with 128kB dm-bufio cache: 12.007
Test: atest google/perf/boottime/boottime-test
Bug: 136247322
Change-Id: I12e342c5692fa89855d9c217f2e94287ff5642a6