swap: convert swapon() to use a folio

Retrieve a folio from the page cache rather than a page.  Saves a couple
of conversions between page & folio.

Link: https://lkml.kernel.org/r/20240826202138.3804238-1-willy@infradead.org


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