scsi: target: add device vendor_id configfs attribute

The vendor_id attribute will allow for the modification of the T10 Vendor
Identification string returned in inquiry responses. Its value can be
viewed and modified via the ConfigFS path at:

target/core/$backstore/$name/wwn/vendor_id

"LIO-ORG" remains the default value, which is set when the backstore device
is enabled.

[mkp: corrected VPD page number]

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>