scsi: target: core: Unify NAA identifier generation

Both the INQUIRY handling and the XCOPY implementation provide functions to
generate an NAA designator. In addition, these functions are poorly named:

 - spc_parse_naa_6h_vendor_specific()
 - target_xcopy_gen_naa_ieee()

Introduce a common NAA 6 designator generation function,
spc_gen_naa_6h_vendor_specific().

Link: https://lore.kernel.org/r/20210420185920.42431-2-s.samoylenko@yadro.com


Signed-off-by: Sergey Samoylenko <s.samoylenko@yadro.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>