Lower the log level for falling back to mkstemp

We recently started trying to use O_TMPFILE without the kernel version
check, we gracefully degrade when it isn't available, and we don't give
operators any way to avoid the O_TMPFILE attempt -- logging at warning
seems way too noisy.

Change-Id: I8f42127fd6eb930282161930c65e8614f3e3175f
Related-Change: I3599e2ab257bcd99467aee83b747939afac639d8

swift/obj/diskfile.py

@@ -1696,7 +1696,7 @@ class BaseDiskFileWriter(object):
                     msg = 'open(%s, O_TMPFILE | O_WRONLY) failed: %s \
                            Falling back to using mkstemp()' \
                            % (self._datadir, os.strerror(err.errno))
-                    self.logger.warning(msg)
+                    self.logger.debug(msg)
                     self.manager.use_linkat = False
                 else:
                     raise

test/unit/obj/test_diskfile.py

@@ -5217,7 +5217,11 @@ class DiskFileMixin(BaseDiskFileTestMixin):
             # Once opening file with O_TMPFILE has failed,
             # failure is cached to not try again
             self.assertFalse(df.manager.use_linkat)
+            # Now that we try to use O_TMPFILE all the time, log at debug
+            # instead of warning
             log = df.manager.logger.get_lines_for_level('warning')
+            self.assertFalse(log)
+            log = df.manager.logger.get_lines_for_level('debug')
             self.assertGreater(len(log), 0)
             self.assertTrue('O_TMPFILE' in log[-1])