Revert "mm: change max readahead size to 512KB"

On low memory targets, we experienced long launch latencies due to the
increase of readahead to 512KB.
Decreasing readahead to 128KB resulted in good improvement in launch
latencies under memory pressure.

This reverts commit 403678af.

Change-Id: I0c00922354920d74d879e7d4bbc38132326ed664
Signed-off-by: Maya Erez <merez@codeaurora.org>