devices/loopback: it is flush_buf not flushbuf

That ironically fix the underlying bug that flush_buf is trying to
fix too, since now an exception is thrown and we are back to auto
clear. The file fd is then closed when the process is terminated.
Anyway, the right fix is to call the correct function.

devices/org.osbuild.loopback

@@ -123,7 +123,7 @@ class LoopbackService(devices.DeviceService):
             # seems to be required when clearing the fd of the
             # loop device (as of kernel 5.13.8) or otherwise
             # it leads to data loss.
-            self.lo.flushbuf()
+            self.lo.flush_buf()
 
             # clear the fd. Since it might not immediately be
             # cleared (due to a race with udev or some other