Remove the hardwired pagefault readaround distance in filemap_nopage() and
use the per-file readahead setting.

The main reason for this is in fact laptop-mode.  If you want to prevent the
disk from spinning up then you want all of your application's pages to be
pulled into memory in one hit.  Otherwise the disk will spin up each time you
use a new part of whatever application(s) you are running.