nfs: Convert nfs to read_folio

This is a "weak" conversion which converts straight back to using pages.
A full conversion should be performed at some point, hopefully by
someone familiar with the filesystem.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

fs/nfs/file.c

@@ -337,7 +337,7 @@ static int nfs_write_begin(struct file *file, struct address_space *mapping,
 	} else if (!once_thru &&
 		   nfs_want_read_modify_write(file, page, pos, len)) {
 		once_thru = 1;
-		ret = nfs_readpage(file, page);
+		ret = nfs_read_folio(file, page_folio(page));
 		put_page(page);
 		if (!ret)
 			goto start;
@@ -514,7 +514,7 @@ static void nfs_swap_deactivate(struct file *file)
 }
 
 const struct address_space_operations nfs_file_aops = {
-	.readpage = nfs_readpage,
+	.read_folio = nfs_read_folio,
 	.readahead = nfs_readahead,
 	.dirty_folio = filemap_dirty_folio,
 	.writepage = nfs_writepage,

fs/nfs/read.c

@@ -333,8 +333,9 @@ readpage_async_filler(struct nfs_readdesc *desc, struct page *page)
  *  -	The error flag is set for this page. This happens only when a
  *	previous async read operation failed.
  */
-int nfs_readpage(struct file *file, struct page *page)
+int nfs_read_folio(struct file *file, struct folio *folio)
 {
+	struct page *page = &folio->page;
 	struct nfs_readdesc desc;
 	struct inode *inode = page_file_mapping(page)->host;
 	int ret;

include/linux/nfs_fs.h

@@ -594,7 +594,7 @@ static inline bool nfs_have_writebacks(const struct inode *inode)
 /*
  * linux/fs/nfs/read.c
  */
-extern int  nfs_readpage(struct file *, struct page *);
+int  nfs_read_folio(struct file *, struct folio *);
 void nfs_readahead(struct readahead_control *);
 
 /*