net: avoid relying on singleflight.Group.DoChan to detect hook called

net: avoid relying on singleflight.Group.DoChan to detect hook called

So next CLs can revert changes to "internal/singleflight" in CL #82795,
then replace it with "golang.org/x/sync/singleflight" instead.

For #31697

Change-Id: I873ce30d7e051539aa6dc5d4f21e558869a6d132
Reviewed-on: https://go-review.googlesource.com/c/go/+/423654
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>