Disable dm-verity hash prefetching.

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.

We've run this change on Pixel 2 since August, and haven't observed any
negative effects. Boot time slightly decreased. With the increased
amount of APEXes we already have, this should now result in an even more
significant boot time decrease.

Bug: 136247322
Test: atest google/perf/boottime/boottime-test
Change-Id: Id588669af1b0b9daaf15323dccf33411e03b8633