scsi_debug: add Capacity Changed Unit Attention

Via sysfs the virtual_gb scsi_debug parameter can be changed while
LUs are in use. If that changes, the 'Capacity data has changed'
Unit Attention is queued on all LUs.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>