scsi: scsi_debug: Rework page code error handling

scsi: scsi_debug: Rework page code error handling

Instead of tracking whether or not the page code is valid in a boolean
variable, jump to error handling code if an unsupported page code is
encountered.

Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240130214911.1863909-15-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>