staging: ion: fix off-by-1 error in heap search

ion allocation was wrong because of an off-by-1 error
in heap search for buffer allocation. Fix it.

Bug: 140507100
Test: ion-unit-tests

Change-Id: I6ac5b0ddc6b31b5409645f62a96781d464b7bdf3
Signed-off-by: Sandeep Patil <sspatil@google.com>