FROMLIST: dm-verity: make real_digest and want_digest fixed-length

Change the digest fields in struct dm_verity_io from variable-length to
fixed-length, since their maximum length is fixed at
HASH_MAX_DIGESTSIZE, i.e. 64 bytes, which is not too big.  This is
simpler and makes the fields a bit faster to access.

(HASH_MAX_DIGESTSIZE did not exist when this code was written, which may
explain why it wasn't used.)

This makes the verity_io_real_digest() and verity_io_want_digest()
functions trivial, but this patch leaves them in place temporarily since
most of their callers will go away in a later patch anyway.

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

Bug: 330611177
Link: https://lore.kernel.org/r/20240621165922.77672-10-ebiggers@kernel.org


Change-Id: Ib9c2a69eb293d8f31fbd7b1c8ff643945b3b6722
Signed-off-by: Eric Biggers <ebiggers@google.com>