verity: remove entry_readahead We don't use entry_readahead and readahead is really best done by other layers. BUG=9752 TEST=Ran platform_DMVerityCorruption on H/W. kernel.git Review URL: http://codereview.chromium.org/6757012 TBRing. Already LGTMed and committed to kernel.git. Change-Id: Id58c0d36fb5576715cbf1e4143a17671b9a6b65b R=wad@chromium.org Review URL: http://codereview.chromium.org/6675058