fix bug in dfs_unmount when test unmount condition found by Rob(rdent@iinet.net.au)

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2356 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/dfs/src/dfs_fs.c

@@ -361,7 +361,7 @@ int dfs_unmount(const char *specialfile)
 	dfs_lock();
 
 	fs = dfs_filesystem_lookup(fullpath);
-	if (fs != RT_NULL && fs->ops->unmount != RT_NULL && fs->ops->unmount(fs) < 0)
+	if (fs == RT_NULL || fs->ops->unmount == RT_NULL || fs->ops->unmount(fs) < 0)
 	{
 		goto err1;
 	}