scsi: target: iscsi: rename some variables to avoid confusion.

This patch renames some variables in chap_server_compute_hash() to make it
harder to confuse the initiator's challenge with the target's challenge
when the mutual chap authentication is used.

Link: https://lore.kernel.org/r/20191017131037.9903-4-mlombard@redhat.com
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>