iomap: Convert iomap_page_create to take a folio

This function already assumed it was being passed a head page, so
just formalise that.

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