ubifs: Fix regression in ubifs_readdir()

commit a00052a2 upstream.

Commit c83ed4c9 ("ubifs: Abort readdir upon error") broke
overlayfs support because the fix exposed an internal error
code to VFS.
Reported-by: Peter Rosin <peda@axentia.se>
Tested-by: Peter Rosin <peda@axentia.se>
Reported-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Tested-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Fixes: c83ed4c9 ("ubifs: Abort readdir upon error")
Signed-off-by: Richard Weinberger <richard@nod.at>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ubifs/dir.c

@@ -448,6 +448,14 @@ static int ubifs_readdir(struct file *file, struct dir_context *ctx)
 out:
 	if (err != -ENOENT)
 		ubifs_err("cannot find next direntry, error %d", err);
+	else
+		/*
+		 * -ENOENT is a non-fatal error in this context, the TNC uses
+		 * it to indicate that the cursor moved past the current directory
+		 * and readdir() has to stop.
+		 */
+		err = 0;
+
 
 	kfree(file->private_data);
 	file->private_data = NULL;