This replaces *traceAllocBlock with traceAllocBlockPtr.

Updates #10600.

Change-Id: I94a20d90f04cca7c457b29062427748e315e4857
Reviewed-on: https://go-review.googlesource.com/17004
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>