UBIFS: Fix possible memory leak in ubifs_readdir()

commit aeeb14f763917ccf639a602cfbeee6957fd944a2 upstream.

If ubifs_tnc_next_ent() returns something else than -ENOENT
we leak file->private_data.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: David Gstir <david@sigma-star.at>
Signed-off-by: Willy Tarreau <w@1wt.eu>