net/http: fix a data race when Request.Cancel is used

net/http: fix a data race when Request.Cancel is used

The "add a Request.Cancel channel" change (https://golang.org/cl/11601)
added support for "race free" cancellation, but introduced a data race. :)

Noticed while running "go test -race net/http". The test is skipped in
short mode, so we never saw it on the dashboard.

Change-Id: Ica14579d8723f8f9d1691e8d56c30b585b332c64
Reviewed-on: https://go-review.googlesource.com/12663
Reviewed-by: Aaron Jacobs <jacobsa@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>