fs: Fix the race condition in file_dup

NULL inode passed to files_allocate doesn't mark file struct in the
allocated state, so other threads which invovle in file allocation
(e.g. open or dup) may allocate the same file struct again.

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: I53ff876eae3c7a1e311e7f671686b73a4b4ef891

include/nuttx/fs/fs.h

@@ -408,7 +408,7 @@ struct file
   int               f_oflags;   /* Open mode flags */
   off_t             f_pos;      /* File position */
   FAR struct inode *f_inode;    /* Driver or file system interface */
-  void             *f_priv;     /* Per file driver private data */
+  FAR void         *f_priv;     /* Per file driver private data */
 };
 
 /* This defines a list of files indexed by the file descriptor */