scsi: target: Add the DUMMY flag to rd_mcp

This commit adds the DUMMY flag to the rd_mcp backend that forces a logical
unit to report itself as not connected device of an unknown type.
Essentially this allows users to create devices identical to the device for
the virtual LUN 0, making it possible to explicitly create a LUN 0 device
and configure its WWNs (e.g. vendor or product name).

Link: https://lore.kernel.org/r/20210322200938.53300-2-k.shelekhin@yadro.com


Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Konstantin Shelekhin <k.shelekhin@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>