iomap: Convert to_iomap_page to take a folio

The big comment about only using a head page can go away now that
it takes a folio argument.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>