quota: add dqi_dirty_list description to comment of Dquot List Management

mainline inclusion
from mainline-v5.3-rc1
commit f44840ad
category: bugfix
bugzilla: 188812,https://gitee.com/openeuler/kernel/issues/I7E0YR
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f44840ad1f822d9ecee6a3f91f2d17825a361307



--------------------------------

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>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>