iomap: factor out a iomap_writepage_handle_eof helper

iomap: factor out a iomap_writepage_handle_eof helper

Most of iomap_do_writepage is dedidcated to handling a folio crossing or
beyond i_size.  Split this is into a separate helper and update the
commens to deal with folios instead of pages and make them more readable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231207072710.176093-6-hch@lst.de
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>