scsi: target: perform t10_wwn ID initialisation in target_alloc_device()

Initialise the t10_wwn vendor, model and revision defaults when a device is
allocated instead of when it's enabled. This ensures that custom vendor or
model strings set prior to enablement are not later overwritten with
default values.

The TRANSPORT_FLAG_PASSTHROUGH conditional can be dropped for the following
reasons:

 - target_core_pscsi overwrites the defaults in the
   pscsi_configure_device() callback.
   + the contents is then only used for ConfigFS via
     $pscsi_dev/statistics/scsi_lu/vend, etc.

 - target_core_user doesn't touch the defaults, nor are they used for
   anything outside of ConfigFS.

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>