dm verity: clean up duplicate hashing code

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

Bug: 21893453
Change-Id: I09c5e81f88ba6a3bce0627f80458ad5571c724d0
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>