runtime: actually fix locking in BenchmarkMSpanCountAlloc
I just submitted CL 255297 which mostly fixed this problem, but totally
forgot to actually acquire/release the heap lock. Oops.
Updates #41391.
Change-Id: I45b42f20a9fc765c4de52476db3654d4bfe9feb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/255298
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>