vmscan: convert the writeback handling in shrink_page_list() to folios

Slightly more efficient due to fewer calls to compound_head().

Link: https://lkml.kernel.org/r/20220504182857.4013401-7-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>