malloc_debug: iterate: use usable_size

malloc_iterate with malloc debug enabled was returning allocation sizes
using the requested size instead of usable size.  If anything wrote
pointers above the requested size but below the usable size, those
pointers would be invisible to libmemunreachable and referenced
allocations could be reported as a leak.

Bug: 27107100
Change-Id: I6b19fd631f68ce93b3aee408cc3d296ec457bd01