iomap: comment on atomic write checks in iomap_dio_bio_iter()

Help explain the code.

Also clarify the comment for bio size check.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20250320120250.4087011-3-john.g.garry@oracle.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>