ion: chunk_heap: fix leak in allocated counter

buffer->size is controlled by the outer ion layer, don't modify it
inside the heap.  Instead, compute the rounded up allocated size
on demand.

Signed-off-by: Colin Cross <ccross@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>