iomap: Convert iomap_releasepage to use a folio

This is an address_space operation, so its argument must remain as a
struct page, but we can use a folio internally.

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