fat: remove ->writepage

->writepage is a very inefficient method to write back data, and only
used through write_cache_pages or a a fallback when no ->migrate_folio
method is present.

Set ->migrate_folio to the generic buffer_head based helper, and remove
the ->writepage implementation.

Link: https://lkml.kernel.org/r/20221202102644.770505-3-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/fat/inode.c

@@ -194,11 +194,6 @@ static int fat_get_block(struct inode *inode, sector_t iblock,
 	return 0;
 }
 
-static int fat_writepage(struct page *page, struct writeback_control *wbc)
-{
-	return block_write_full_page(page, fat_get_block, wbc);
-}
-
 static int fat_writepages(struct address_space *mapping,
 			  struct writeback_control *wbc)
 {
@@ -346,12 +341,12 @@ static const struct address_space_operations fat_aops = {
 	.invalidate_folio = block_invalidate_folio,
 	.read_folio	= fat_read_folio,
 	.readahead	= fat_readahead,
-	.writepage	= fat_writepage,
 	.writepages	= fat_writepages,
 	.write_begin	= fat_write_begin,
 	.write_end	= fat_write_end,
 	.direct_IO	= fat_direct_IO,
-	.bmap		= _fat_bmap
+	.bmap		= _fat_bmap,
+	.migrate_folio	= buffer_migrate_folio,
 };
 
 /*