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
2 files changed