ext4: return correct wbc.nr_to_write in ext4_da_writepages

When ext4_da_writepages increases the nr_to_write in writeback_control
then it must always re-base the return value.  Originally there was a
(misguided) attempt prevent wbc.nr_to_write from going negative.  In
fact, it's necessary to allow nr_to_write to be negative so that
wb_writeback() can correctly calculate how many pages were actually
written.  

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -2967,8 +2967,7 @@ static int ext4_da_writepages(struct address_space *mapping,
 out_writepages:
 	if (!no_nrwrite_index_update)
 		wbc->no_nrwrite_index_update = 0;
-	if (wbc->nr_to_write > nr_to_writebump)
-		wbc->nr_to_write -= nr_to_writebump;
+	wbc->nr_to_write -= nr_to_writebump;
 	wbc->range_start = range_start;
 	trace_ext4_da_writepages_result(inode, wbc, ret, pages_written);
 	return ret;