configfs: fix kernel-doc markup issue

Add explanation for 'frag' parameter to avoid kernel-doc issue:
fs/configfs/dir.c:277: warning: Function parameter or member 'frag' not
described in 'configfs_create_dir'

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>

fs/configfs/dir.c

@@ -267,6 +267,7 @@ static void configfs_remove_dirent(struct dentry *dentry)
  *	configfs_create_dir - create a directory for an config_item.
  *	@item:		config_itemwe're creating directory for.
  *	@dentry:	config_item's dentry.
+ *	@frag:		config_item's fragment.
  *
  *	Note: user-created entries won't be allowed under this new directory
  *	until it is validated by configfs_dir_set_ready()