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>