iomap: rename iomap_writepage_map to iomap_writeback_folio

->writepage is gone, and our naming wasn't always that great to start
with.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/20250710133343.399917-9-hch@lst.de


Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>