iomap: Convert iomap_add_to_ioend() to take a folio

We still iterate one block at a time, but now we call compound_head()
less often.

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