scsi_debug: Implement WRITE BUFFER command

Accept the WRITE BUFFER command and do nothing other than
set the appropriate "microcode has been changed" UA on the LU.

>From an earlier patch by Doug Gilbert.

Signed-off-by: Ewan D. Milne <emilne@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Tested-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>