Fix the comments to help people understand the code.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
[darrick: fix the indenting problems too]
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>