ubifs: Implement ubifs_lpt_lookup using ubifs_pnode_lookup

ubifs_lpt_lookup() starts by looking up the nth pnode in the LPT. We
already have this functionality in ubifs_pnode_lookup(). Use this
function rather than open coding its functionality.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>