Replace file.flush() calls with flush_sync() helper

Calls to `os.fsync(f.fileno())` need to be accompained by `f.flush()`.

Commit 8bbeedc93f introduces the helper
`ipapython.ipautil.flush_sync()`, which handles all calls in the right
order.

However, `flush_sync()` takes as parameter a file object with fileno
and name, where name must be a path to the file, this isn't possible
in some cases where file descriptors are used.

Issue: https://pagure.io/freeipa/issue/7251

Signed-off-by: Armando Neto <abiagion@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipaclient/install/client.py

@@ -1274,10 +1274,9 @@ def do_nsupdate(update_txt):
     logger.debug("Writing nsupdate commands to %s:", UPDATE_FILE)
     logger.debug("%s", update_txt)
 
-    update_fd = open(UPDATE_FILE, "w")
-    update_fd.write(update_txt)
-    update_fd.flush()
-    update_fd.close()
+    with open(UPDATE_FILE, "w") as f:
+        f.write(update_txt)
+        ipautil.flush_sync(f)
 
     result = False
     try: