scsi: target: Fix iscsi transport id buf len calculation

The isid returned to the initiator is in string format which is 12
bytes. We also only add 1 terminating NULL and not one after the initiator
name and another one after the isid.

Link: https://lore.kernel.org/r/1593654203-12442-7-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>