vmscan: convert dirty page handling to folios

Mostly this just eliminates calls to compound_head(), but
NR_VMSCAN_IMMEDIATE was being incremented by 1 instead of by nr_pages.

Link: https://lkml.kernel.org/r/20220504182857.4013401-10-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>