nvme-auth: use crypto_shash_tfm_digest()

Simplify nvme_auth_augmented_challenge() by using
crypto_shash_tfm_digest() instead of an alloc+init+update+final
sequence.  This should also improve performance.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>