DFS/jffs2: fix file mode init bug in jffs2_open/opendir

found by haitao52198
URL:http://www.rt-thread.org/phpBB3/viewtopic.php?f=3&t=3112&p=17153#p17153
DFS/jffs2: fix bug when umount jffs2

found by haitao5198
clear mount parition device talbe when jffs2 mount falied
Merge pull request #165 from prife/jffs2-fix

DFS/jffs2: fix file mode init bug in jffs2_open/opendir