mm: change max readahead size to 512KB

Change the VM_MAX_READAHEAD value from the default 128KB
to 512KB. This will allow the readahead window to grow to a maximum size
of 512KB, which greatly benefits to sequential read throughput.

Change-Id: Ia0780ea4e2a4ae0b6111485b72fb25376dcb1f96
Signed-off-by: Lee Susman <lsusman@codeaurora.org>