Correct dfs fd_is_open search index error

Author: AubrCool

components/dfs/src/dfs.c

@@ -234,7 +234,11 @@ int fd_is_open(const char *pathname)
             mountpath = fullpath + strlen(fs->path);
 
         dfs_lock();
+#ifdef DFS_USING_STDIO
+        for (index = 3; index < DFS_FD_MAX+3; index++)
+#else
         for (index = 0; index < DFS_FD_MAX; index++)
+#endif
         {
             fd = &(fd_table[index]);
             if (fd->fs == RT_NULL)