iomap: add comments for ifs_clear/set_range_dirty()

iomap: add comments for ifs_clear/set_range_dirty()

The range alignment strategy differs between ifs_clear_range_dirty() and
ifs_set_range_dirty(). The former rounds inwards to clear only
fully-covered blocks, while the latter rounds outwards to mark any
partially-touched block as dirty. Add comments to document this
asymmetry in block range calculation.

Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20260714082325.325163-6-yi.zhang@huaweicloud.com
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>