dfs: rename varialbe name in dfs_filesystem_lookup

empty is confused in dfs_filesystem_lookup.
dfs: fix dfs_unmount bug

fix #211
dfs: make elmfatfs check secter size everytime

more infomation:
http://www.rt-thread.org/phpBB3/topic2965.html
Merge pull request #215 from prife/master

fix dfs_unmount bug