[scsi] Generalise iscsi_detached_command() to scsi_detached_command()

src/drivers/block/scsi.c

 	return container_of ( blockdev, struct scsi_device, blockdev );
 }
 
+/**
+ * Handle SCSI command with no backing device
+ *
+ * @v scsi		SCSI device
+ * @v command		SCSI command
+ * @ret rc		Return status code
+ */
+int scsi_detached_command ( struct scsi_device *scsi __unused,
+			    struct scsi_command *command __unused ) {
+	return -ENODEV;
+}
+
 /**
  * Issue SCSI command
  *

src/include/gpxe/scsi.h

 	struct refcnt *backend;
 };
 
+extern int scsi_detached_command ( struct scsi_device *scsi,
+				   struct scsi_command *command );
 extern int init_scsidev ( struct scsi_device *scsi );
 
 #endif /* _GPXE_SCSI_H */

src/net/tcp/iscsi.c

 	return 0;
 }
 
-static int iscsi_detached_command ( struct scsi_device *scsi __unused,
-				    struct scsi_command *command __unused ) {
-	return -ENODEV;
-}
-
 /**
  * Shut down iSCSI interface
  *
 	xfer_nullify ( &iscsi->socket );
 	iscsi_close_connection ( iscsi, 0 );
 	process_del ( &iscsi->process );
-	scsi->command = iscsi_detached_command;
+	scsi->command = scsi_detached_command;
 	ref_put ( scsi->backend );
 	scsi->backend = NULL;
 }