mm/vmscan: convert isolate_lru_pages() to use a folio

Remove a few hidden calls to compound_head, saving 279 bytes of text.

Link: https://lkml.kernel.org/r/20220617154248.700416-3-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>