alinux: jbd2/doc: fix new kernel-doc warning

Fix the following kernel-doc warning:

include/linux/jbd2.h:1184: warning: Function parameter or member 'j_checkpoint_task' not described in 'journal_s'

Fixes: 3999cdd9 ("alinux: jbd2: create jbd2-ckpt thread for journal checkpoint")
Reported-by: Nkbuild test robot <lkp@intel.com>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>

include/linux/jbd2.h

@@ -1118,7 +1118,11 @@ struct journal_s
 	void			(*j_commit_callback)(journal_t *,
 						     transaction_t *);
 
-	/* Pointer to the current checkpoint thread for this journal */
+	/**
+	 * @j_checkpoint_task:
+	 *
+	 * Pointer to the current checkpoint thread for this journal.
+	 */
 	struct task_struct	*j_checkpoint_task;
 
 	/*