dm-verity: make verity_hash() take dm_verity_io instead of ahash_request

In preparation for adding shash support to dm-verity, change
verity_hash() to take a pointer to a struct dm_verity_io instead of a
pointer to the ahash_request embedded inside it.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>