crypto: x86/crct10dif-pcl - fix use via crypto_shash_digest()

The ->digest() method of crct10dif-pclmul reads the current CRC value
from the shash_desc context.  But this value is uninitialized, causing
crypto_shash_digest() to compute the wrong result.  Fix it.

Probably this wasn't noticed before because lib/crc-t10dif.c only uses
crypto_shash_update(), not crypto_shash_digest().  Likewise,
crypto_shash_digest() is not yet tested by the crypto self-tests because
those only test the ahash API which only uses shash init/update/final.

Fixes: 0b95a7f8 ("crypto: crct10dif - Glue code to cast accelerated CRCT10DIF assembly as a crypto transform")
Cc: <stable@vger.kernel.org> # v3.11+
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>