[internal-branch.go1.16-vendor] http2: on write errors, close ClientConn before returning from RoundTrip

Deflakes TestTransportRoundtripCloseOnWriteError.

Updates golang/go#49076

Change-Id: I4384d9091d55307d15fbd44b1b8137dcc8939c86
Reviewed-on: https://go-review.googlesource.com/c/net/+/356029
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/net/+/357097
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: neild

http2/transport.go

@@ -1332,14 +1332,15 @@ func (cs *clientStream) cleanupWriteRequest(err error) {
 	if cs.ID != 0 {
 		cc.forgetStreamID(cs.ID)
 	}
-	close(cs.donec)
 
 	cc.wmu.Lock()
 	werr := cc.werr
 	cc.wmu.Unlock()
 	if werr != nil {
 		cc.Close()
 	}
+
+	close(cs.donec)
 }
 
 // awaitOpenSlotForStream waits until len(streams) < maxConcurrentStreams.