mm: convert swap_readpage() to swap_read_folio()

All callers have a folio, so pass it in, saving two calls to
compound_head().

Link: https://lkml.kernel.org/r/20231213215842.671461-11-willy@infradead.org


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