They aren't really races, or at least they don't have any
observable effect. The spec is silent on whether these are actually
races or not.

Fix this problem by not using the address of len (or of cap)
as the location where channel operations are recorded to occur.
Use a random other field of hchan for that.

I'm not 100% sure we should in fact fix this. Opinions welcome.

Fixes #27070

Change-Id: Ib4efd4b62e0d1ef32fa51e373035ef207a655084
Reviewed-on: https://go-review.googlesource.com/135698Reviewed-by: Dmitry Vyukov <dvyukov@google.com>