UPSTREAM: dm verity: clean up duplicate hashing code

(cherry-picked from 6dbeda3469ced777bc3138ed5918f7ae79670b7b)

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

Bug: 21893453
Change-Id: Ic7fd1c714554509d5b1583630de45f378c3360d6
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>