Fix flush and fsync in xmp.py.

- Don't call file.flush() if file is not open for writing
  (a fix for OS-es which adhere more to POSIX than Linux in this respect,
  eg. *BSD).
- Do call file.flush() both from both of fsync and flush methods because data
  is to be flushed out from userspace cache anyway before we can go on.

Author: dzsekijo

example/xmp.py

@@ -180,14 +180,19 @@ def write(self, buf, offset):
         def release(self, flags):
             self.file.close()
 
+        def _fflush(self):
+            if 'w' in self.file.mode or 'a' in self.file.mode:
+                self.file.flush()
+
         def fsync(self, isfsyncfile):
+            self._fflush()
             if isfsyncfile and hasattr(os, 'fdatasync'):
                 os.fdatasync(self.fd)
             else:
                 os.fsync(self.fd)
 
         def flush(self):
-            self.file.flush()
+            self._fflush()
             # cf. xmp_flush() in fusexmp_fh.c
             os.close(os.dup(self.fd))