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

commit 25c3d30c upstream.

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>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>