scsi: target: Fix iscsi transport id parsing

The length passed in the ADDITIONAL LENGTH field includes padding and the
terminating NULL for the last field (name or isid depending on the format),
so we should not also try to calculate that and then double add that to the
returned length.

Link: https://lore.kernel.org/r/1593654203-12442-5-git-send-email-michael.christie@oracle.com


Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>