scsi: target: iscsi: tie the challenge length to the hash digest size

Link: https://lore.kernel.org/r/20191017131037.9903-3-mlombard@redhat.com
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Tested-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>