System will crash when the romfs is erased. Add checks before using them
to avoid it.

Clang complains about "Null pointer argument in call to string
comparison function". So assert on it.

This issue causes fstat & lseek not correct.

I have to mention that this bug is introduced by myself, in the last
commit <fd07f30f> to dfs_nfs.c

more infomation:
http://www.rt-thread.org/phpBB3/topic2965.html

fix #211

empty is confused in dfs_filesystem_lookup.

clean code with pointer rather than index

decrease indentation to make cleaner code
fix mkfs bug when there is no mkfs implementation

_TIME_T_DEFINED was pre-defined in order to build jffs2 with VC.
While it may cause many side effects.

found by haitao5198
clear mount parition device talbe when jffs2 mount falied

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