net/http: fix data race due to writeLoop goroutine left running

Fix a data race for clients that mutate requests after receiving a
response error which is caused by the writeLoop goroutine left
running, this can be seen on canceled requests.

Fixes #37669

Change-Id: I0e0e4fd63266326b32587d8596456760bf848b13
Reviewed-on: https://go-review.googlesource.com/c/go/+/232799


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>