loop: add direct-io support

Support the LOOP_SET_DIRECT_IO ioctl, which alows us to control
whether or not a loopback device should perform its own buffering
or rely on the one done by the underlying backing file.

Enabling this should improve both throughput and memory consumption,
it is not currently hooked up as more testing would be required.

osbuild/loop.py

@@ -207,6 +207,20 @@ class Loop:
                 info.lo_flags &= ~self.LO_FLAGS_PARTSCAN
         fcntl.ioctl(self.fd, self.LOOP_SET_STATUS64, info)
 
+    def set_direct_io(self, dio=True):
+        """Set the direct-IO property on the loopback device
+
+        Enabling direct IO allows one to avoid double caching, which
+        should improve performance and memory usage.
+
+        Parameters
+        ----------
+        dio : bool, optional
+            Whether or not to enable direct IO (default is True)
+        """
+
+        fcntl.ioctl(self.fd, self.LOOP_SET_DIRECT_IO, dio)
+
     def mknod(self, dir_fd=None, mode=0o600):
         """Create a secondary device node