[release-branch.go1.15] net/http: add connections back that haven't been canceled

Issue #41600 fixed the issue when a second request canceled a connection
while the first request was still in roundTrip.
This uncovered a second issue where a request was being canceled (in
roundtrip) but the connection was put back into the idle pool for a
subsequent request.
The fix is the similar except its now in readLoop instead of roundTrip.
A persistent connection is only added back if it successfully removed
the cancel function; otherwise we know the roundTrip has started
cancelRequest.

Fixes #42935.
Updates #42942.

Change-Id: Ia56add20880ccd0c1ab812d380d8628e45f6f44c
Reviewed-on: https://go-review.googlesource.com/c/go/+/274973


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
(cherry picked from commit 854a2f8e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/297910
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>