scsi: target: Fix iscsi transport id buffer setup

This fixes the following bugs with the transport id setup for iscsi:

 1. Incorrectly adding NULL after initiator name for TPID format 1.

 2. For TPID format 1 buffer setup we are doing off+len, off++ and then
    also len+=some_value. This results in the isid going past buffer
    boundaries when we then do buf[off+len]

 3. The pr_reg_isid is the isid in string format which is 12 bytes, but we
    are only copying 6 bytes.

Link: https://lore.kernel.org/r/1593654203-12442-6-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>