crypto: sha512 - Fix byte counter overflow in SHA-512

The current code only increments the upper 64 bits of the SHA-512 byte
counter when the number of bytes hashed happens to hit 2^64 exactly.

This patch increments the upper 64 bits whenever the lower 64 bits
overflows.

Signed-off-by: Kent Yoder <key@linux.vnet.ibm.com>
Cc: stable@kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>