Allow callers to iterate over each folio instead of each page.  The
bio need not have been constructed using folios originally.
Signed-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: NJens Axboe <axboe@kernel.dk>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDarrick J. Wong <djwong@kernel.org>