mm/lru: Convert __pagevec_lru_add_fn to take a folio

This saves five calls to compound_head(), totalling 60 bytes of text.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>