ubifs: lpt: Fix wrong pnode number range in comment

The comment above pnode_lookup claims the range for the pnode number is
from 0 to main_lebs - 1. This is wrong because every pnode has
informations about UBIFS_LPT_FANOUT LEBs, thus the corrent range is
0 to to (main_lebs - 1) / UBIFS_LPT_FANOUT.

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