UPSTREAM: dm verity: clean up duplicate hashing code

Handle dm-verity salting in one place to simplify the code.

Change-Id: If923a01dc63ae5123af13ba1b0863b73e33ddf46
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit 6dbeda3469ced777bc3138ed5918f7ae79670b7b)