ext4: clear buffer_uninit flag when submitting IO

Currently noone cleared buffer_uninit flag. This results in writeback
needlessly marking io_end as needing extent conversion scanning extent
tree for extents to convert. So clear the buffer_uninit flag once the
buffer is submitted for IO and the flag is transformed into
EXT4_IO_END_UNWRITTEN flag.
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: NZheng Liu <wenqing.lz@taobao.com>

fs/ext4/page-io.c

@@ -387,7 +387,7 @@ static int io_submit_add_bh(struct ext4_io_submit *io,
 	if (ret != bh->b_size)
 		goto submit_and_retry;
 	io_end = io->io_end;
-	if (buffer_uninit(bh))
+	if (test_clear_buffer_uninit(bh))
 		ext4_set_io_unwritten_flag(inode, io_end);
 	io_end->size += bh->b_size;
 	io->io_next_block++;