tweak dokan file truncating

fs/expose/dokan/__init__.py

@@ -512,8 +512,10 @@ class FSOperations(object):
         lock.acquire()
         try:
             pos = file.tell()
+            if length != pos:
                 file.seek(length)
             file.truncate()
+            if pos < length:
                 file.seek(min(pos,length))
         finally:
             lock.release()