modify dns cache logic

Author: YangSen-qn

library/src/main/java/com/qiniu/android/storage/persistent/DnsCacheFile.java

@@ -21,16 +21,16 @@ public DnsCacheFile(String directory) throws IOException {
         this.directory = directory;
         f = new File(directory);
 
-        if (!f.isDirectory()) {
-            throw new IOException("does not mkdir");
-        }
-
         if (!f.exists()) {
             boolean r = f.mkdirs();
             if (!r) {
                 throw new IOException("mkdir failed");
             }
         }
+
+        if (!f.isDirectory()) {
+            throw new IOException("does not mkdir");
+        }
     }
 
     /**