Wrong dir is created when using mult-dirs storage.

[cserwen]

BUG REPORT

1. Please describe the issue you observed:

• What did you do (The steps to reproduce)?
  Test multi-disk storage RIP-7 Multiple Directories Storage Support #3357

• What did you see?
  When I set storePathCommitLog as /home/work/ssd2/rocketmq/commitlog/,/home/work/ssd3/rocketmq/commitlog/,/home/work/ssd4/rocketmq/commitlog/, the broker will create this directory：/home/work/ssd2/rocketmq/commitlog/,/home/work/ssd3/rocketmq/commitlog/,/home/work/ssd4/rocketmq/commitlog/

[work@localhost ssd2]$ pwd
/home/work/ssd2/rocketmq/commitlog/,/home/work/ssd3/rocketmq/commitlog/,/home/work/ssd4/rocketmq/commitlog

• What did you expect to see?
  This directory should not be created

2. Please tell us about your environment:
   Branch：master
   OS：Linux

3. Other information (e.g. detailed explanation, logs, related issues, suggestions how to fix, etc):
   I find it will be created here:
   

   rocketmq/store/src/main/java/org/apache/rocketmq/store/DefaultMessageStore.java

   Line 174 in 799d412

   MappedFile.ensureDirOK(getStorePathPhysic());

   
   This method does not consider the multi-directory case:
   

   rocketmq/store/src/main/java/org/apache/rocketmq/store/MappedFile.java

   Line 81 in 799d412

   public static void ensureDirOK(final String dirName) {