scsi: target: Add device product id and revision configfs attributes

The product_id and revision attributes will allow for the modification of
the T10 Model and Revision strings returned in inquiry responses.  Its
value can be viewed and modified via the ConfigFS path at:

target/core/$backstore/$name/wwn/product_id
target/core/$backstore/$name/wwn/revision

[mkp: dropped parentheses as requested by Bart]

Signed-off-by: Alan Adamson <alan.adamson@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>