mm: pass a folio to swap_readpage_bdev_sync()

Make it plain that this takes the head page (which before this point
was just an assumption, but is now enforced by the compiler).

Link: https://lkml.kernel.org/r/20231213215842.671461-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>