scsi: target: iscsi: Add support for extended CDB AHS

Variable length SCSI commands are transferred over iSCSI via two CDB
buffers - in Basic Header Segment and in Additional Header Segment (AHS).
Since AHS is not supported yet, a target reads just BHS (48 byte) from TCP
and treats the remaining octets as a next new iSCSI PDU that causes
protocol errors.

Add support for the Extended CDB AHS type.

Link: https://lore.kernel.org/r/20220718152555.17084-2-d.bogdanov@yadro.com


Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>