dm-verity: reduce scope of real and wanted digests

dm-verity: reduce scope of real and wanted digests

In preparation for supporting interleaved hashing where dm-verity will
need to keep track of the real and wanted digests for multiple data
blocks simultaneously, stop using the want_digest and real_digest fields
of struct dm_verity_io from so many different places.  Specifically:

- Make various functions take want_digest as a parameter rather than
  having it be implicitly passed via the struct dm_verity_io.

- Add a new tmp_digest field, and use this instead of real_digest when
  computing a digest solely for the purpose of immediately checking it.

The result is that real_digest and want_digest are used only by
verity_verify_io().

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>