nfs: Convert to migrate_folio

Use a folio throughout this function.  migrate_page() will be converted
later.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>