scsi_debug: append inject error flags onto scsi_cmnd object

The way the existing scsi_debug command parser associated various
inject error flags to a command was difficult to replicate in the
table driven parser. This patch adds infrastructure to append those
flags to the end of a scsi_cmnd object with the cmd_size host
template option.

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