[release-branch.go1.25] runtime: lock mheap_.speciallock when allocating synctest specials
Avoid racing use of mheap_.specialBubbleAlloc.
For #75134
Fixes #75347
Change-Id: I0c9140c18d2bca1e1c3387cd81230f0e8c9ac23e
Reviewed-on: https://go-review.googlesource.com/c/go/+/699255
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit
5dcedd65504cc9cadc9a5ea8bc3af51a26eec704)
Reviewed-on: https://go-review.googlesource.com/c/go/+/701797
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>