Prevent filesystem thrashing in low-mem situations by turning off
read-ahead. Access patterns of baloo are unlikely to benefit from
read-ahead and preserving cache is more valuable than reading ahead
potentially unused data.
In low-memory conditions, the random access patterns of Baloo combined
with read-ahead tend to dominate the cache. Let Baloo be the slow player
here, and leave system cache and memory for interactive performance.
Depends on D24539