vmscan: convert page buffer handling to use folios

This mostly just removes calls to compound_head() although nr_reclaimed
should be incremented by the number of pages, not just 1.

Link: https://lkml.kernel.org/r/20220504182857.4013401-11-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>