crypto: shash - fix missed optimization in shash_ahash_digest()

shash_ahash_digest(), which is the ->digest() method for ahash tfms that
use an shash algorithm, has an optimization where crypto_shash_digest()
is called if the data is in a single page.  But an off-by-one error
prevented this path from being taken unless the user happened to provide
extra data in the scatterlist.  Fix it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>