scsi: scsi_debug: Fix missing error code in scsi_debug_init()

Smatch reports: drivers/scsi/scsi_debug.c:6996
	scsi_debug_init() warn: missing error code 'ret'

Although it is unlikely that KMEM_CACHE might fail, but if it does then ret
might be zero. So to fix this explicitly mark ret as "-ENOMEM" and then
goto driver_unreg.

Fixes: 1107c7b2

 ("scsi: scsi_debug: Dynamically allocate sdebug_queued_cmd")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Link: https://lore.kernel.org/r/20230406074607.3637097-1-harshit.m.mogalapalli@oracle.com
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>