[SCSI] iscsi: fix 2.6.19 data digest calculation bug

The transition from crypto_digest_*() to  the crypto_hash_*() family
introduced a bug into the data digest calculation: crypto_hash_update() is
called with the number of S/G elements instead of the S/G lists data size.

Signed-off-by: Arne Redlich <arne.redlich@xiranet.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>