runtime: fix leak and locking in BenchmarkMSpanCountAlloc

CL 249917 made the mspan in MSpanCountAlloc no longer stack-allocated
(for good reason), but then allocated an mspan on each call and did not
free it, resulting in a leak. That allocation was also not protected by
the heap lock, which could lead to data corruption of mheap fields and
the spanalloc.

To fix this, export some functions to allocate/free dummy mspans from
spanalloc (with proper locking) and allocate just one up-front for the
benchmark, freeing it at the end. Then, update MSpanCountAlloc to accept
a dummy mspan.

Note that we need to allocate the dummy mspan up-front otherwise we
measure things like heap locking and fixalloc performance instead of
what we actually want to measure: how fast we can do a popcount on the
mark bits.

Fixes #41391.

Change-Id: If6629a6ec1ece639c7fb78532045837a8c872c04
Reviewed-on: https://go-review.googlesource.com/c/go/+/255297
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
2 files changed
tree: d9cc778fe5ef08e0b20391d52fce199e4d674b80
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  10. SECURITY.md
  11. api/
  12. doc/
  13. favicon.ico
  14. lib/
  15. misc/
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