kernfs: add kernfs_open_file->priv

Add a private data field to be used by kernfs file operations.  This
generally makes sense and will be used by cgroup.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kernfs.h

@@ -146,6 +146,7 @@ struct kernfs_open_file {
 	/* published fields */
 	struct kernfs_node	*kn;
 	struct file		*file;
+	void			*priv;
 
 	/* private fields, do not use outside kernfs proper */
 	struct mutex		mutex;