nfs: Remove writepage

NFS already has writepages and migrate_folio, so it does not need to
implement writepage. The writepage operation is deprecated as it leads
to worse performance under high memory pressure due to folios being
written out in LRU order rather than sequentially within a file.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

fs/nfs/file.c

···
 	.read_folio = nfs_read_folio,
 	.readahead = nfs_readahead,
 	.dirty_folio = filemap_dirty_folio,
-	.writepage = nfs_writepage,
 	.writepages = nfs_writepages,
 	.write_begin = nfs_write_begin,
 	.write_end = nfs_write_end,

fs/nfs/write.c

···
 	return err;
 }
 
-int nfs_writepage(struct page *page, struct writeback_control *wbc)
-{
-	struct folio *folio = page_folio(page);
-	int ret;
-
-	ret = nfs_writepage_locked(folio, wbc);
-	if (ret != AOP_WRITEPAGE_ACTIVATE)
-		unlock_page(page);
-	return ret;
-}
-
 static int nfs_writepages_callback(struct folio *folio,
 				   struct writeback_control *wbc, void *data)
 {

include/linux/nfs_fs.h

···
  * linux/fs/nfs/write.c
  */
 extern int  nfs_congestion_kb;
-extern int  nfs_writepage(struct page *page, struct writeback_control *wbc);
 extern int  nfs_writepages(struct address_space *, struct writeback_control *);
 extern int  nfs_flush_incompatible(struct file *file, struct folio *folio);
 extern int  nfs_update_folio(struct file *file, struct folio *folio,