UBI: fix rb_tree node comparison in add_map

The comparisons used in add_vol() shouldn't be identical.  Pretty sure
the following is correct but it is completely untested.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/fastmap.c

@@ -125,9 +125,9 @@ static struct ubi_ainf_volume *add_vol(struct ubi_attach_info *ai, int vol_id,
 		parent = *p;
 		av = rb_entry(parent, struct ubi_ainf_volume, rb);
 
-		if (vol_id > av->vol_id)
+		if (vol_id < av->vol_id)
 			p = &(*p)->rb_left;
-		else if (vol_id > av->vol_id)
+		else
 			p = &(*p)->rb_right;
 	}