iomap: Convert iomap_migrate_page() to use folios

The arguments are still pages for now, but we can use folios internally
and cut out a lot of calls to compound_head().

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