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.

Change-Id: I288ffc1221ce96cfe2591468502ac3279065bde4
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: f526a55b400d166dafc6062e5818a2410555ef39
Git-repo: https://android.googlesource.com/kernel/common/
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>