verity: statically allocate root_digest

BUG=chromium-os:9752
TEST=Ran dm-verity.git unit tests. Ran platform_DMVerityCorruption on H/W.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
kernel.git Reviewed-on: http://gerrit.chromium.org/gerrit/4466

Code has already been reviewed and committed to kernel.git.

Change-Id: I042941f480f1faf8063dbd4424b5d3b7c1978a9a
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/6776
2 files changed