f2fs: change the timing of f2fs_wait_on_page_writeback

some backing devices need pages to be stable during writeback. It doesn't
matter if
the page is completely overwritten or already uptodate, it needs to wait
before write.
Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1383,13 +1383,13 @@ static int f2fs_write_begin(struct file *file, struct address_space *mapping,
 	f2fs_put_dnode(&dn);
 	f2fs_unlock_op(sbi);
 
+	f2fs_wait_on_page_writeback(page, DATA);
+
 	if (len == PAGE_CACHE_SIZE)
 		goto out_update;
 	if (PageUptodate(page))
 		goto out_clear;
 
-	f2fs_wait_on_page_writeback(page, DATA);
-
 	if ((pos & PAGE_CACHE_MASK) >= i_size_read(inode)) {
 		unsigned start = pos & (PAGE_CACHE_SIZE - 1);
 		unsigned end = start + len;