devices/loopback: tolerate existing device node

It is not an error if the device node already exist, which is e.g.
the case when we are using `/dev` of the host.

devices/org.osbuild.loopback

@@ -18,11 +18,13 @@ documentation for `osbuil.util.udev.UdevInhibitor`.
 
 
 import argparse
+import errno
 import os
 import sys
 from typing import Dict
 
 from osbuild import devices, loop
+from osbuild.util import ctx
 from osbuild.util.udev import UdevInhibitor
 
 SCHEMA = """
@@ -106,7 +108,8 @@ class LoopbackService(devices.DeviceService):
         dir_fd = -1
         try:
             dir_fd = os.open(devpath, os.O_CLOEXEC | os.O_PATH)
-            self.lo.mknod(dir_fd)
+            with ctx.suppress_oserror(errno.EEXIST):
+                self.lo.mknod(dir_fd)
         finally:
             if dir_fd > -1:
                 os.close(dir_fd)