nfs: Convert from launder_page to launder_folio

We don't need to use page_file_mapping() here because launder_folio
is never called for swap cache pages.  We also don't need to
cast an loff_t in order to print it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Acked-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Tested-by: Mike Marshall <hubcap@omnibond.com> # orangefs
Tested-by: David Howells <dhowells@redhat.com> # afs