dm crypt: convert essiv from ahash to shash

dm crypt: convert essiv from ahash to shash

In preparing to remove all stack VLA usage from the kernel[1], remove
the discouraged use of AHASH_REQUEST_ON_STACK in favor of the smaller
SHASH_DESC_ON_STACK by converting from ahash-wrapped-shash to direct
shash.  The stack allocation will be made a fixed size in a later patch
to the crypto subsystem.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>