ocfs2: _really_ sync the right range

commit 64b4e252 upstream.

"ocfs2 syncs the wrong range" had been broken; prior to it the
code was doing the wrong thing in case of O_APPEND, all right,
but _after_ it we were syncing the wrong range in 100% cases.
*ppos, aka iocb->ki_pos is incremented prior to that point,
so we are always doing sync on the area _after_ the one we'd
written to.

Spotted by Joseph Qi <joseph.qi@huawei.com> back in January;
unfortunately, I'd missed his mail back then ;-/
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/ocfs2/file.c

@@ -2375,10 +2375,14 @@ static ssize_t ocfs2_file_write_iter(struct kiocb *iocb,
 	/* buffered aio wouldn't have proper lock coverage today */
 	BUG_ON(ret == -EIOCBQUEUED && !(file->f_flags & O_DIRECT));
 
+	if (unlikely(written <= 0))
+		goto no_sync;
+
 	if (((file->f_flags & O_DSYNC) && !direct_io) || IS_SYNC(inode) ||
 	    ((file->f_flags & O_DIRECT) && !direct_io)) {
-		ret = filemap_fdatawrite_range(file->f_mapping, *ppos,
-					       *ppos + count - 1);
+		ret = filemap_fdatawrite_range(file->f_mapping,
+					       iocb->ki_pos - written,
+					       iocb->ki_pos - 1);
 		if (ret < 0)
 			written = ret;
 
@@ -2391,10 +2395,12 @@ static ssize_t ocfs2_file_write_iter(struct kiocb *iocb,
 		}
 
 		if (!ret)
-			ret = filemap_fdatawait_range(file->f_mapping, *ppos,
-						      *ppos + count - 1);
+			ret = filemap_fdatawait_range(file->f_mapping,
+						      iocb->ki_pos - written,
+						      iocb->ki_pos - 1);
 	}
 
+no_sync:
 	/*
 	 * deep in g_f_a_w_n()->ocfs2_direct_IO we pass in a ocfs2_dio_end_io
 	 * function pointer which is called when o_direct io completes so that