quota: add dqi_dirty_list description to comment of Dquot List Management

Actually there are four lists for dquot management, so add
the description of dqui_dirty_list to comment.
Signed-off-by: Chengguang Xu <cgxu519@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/dquot.c

@@ -223,9 +223,9 @@ static void put_quota_format(struct quota_format_type *fmt)
 
 /*
  * Dquot List Management:
- * The quota code uses three lists for dquot management: the inuse_list,
- * free_dquots, and dquot_hash[] array. A single dquot structure may be
- * on all three lists, depending on its current state.
+ * The quota code uses four lists for dquot management: the inuse_list,
+ * free_dquots, dqi_dirty_list, and dquot_hash[] array. A single dquot
+ * structure may be on some of those lists, depending on its current state.
  *
  * All dquots are placed to the end of inuse_list when first created, and this
  * list is used for invalidate operation, which must look at every dquot.
@@ -236,6 +236,11 @@ static void put_quota_format(struct quota_format_type *fmt)
  * dqstats.free_dquots gives the number of dquots on the list. When
  * dquot is invalidated it's completely released from memory.
  *
+ * Dirty dquots are added to the dqi_dirty_list of quota_info when mark
+ * dirtied, and this list is searched when writing dirty dquots back to
+ * quota file. Note that some filesystems do dirty dquot tracking on their
+ * own (e.g. in a journal) and thus don't use dqi_dirty_list.
+ *
  * Dquots with a specific identity (device, type and id) are placed on
  * one of the dquot_hash[] hash chains. The provides an efficient search
  * mechanism to locate a specific dquot.