http2: on write errors, close ClientConn before returning from RoundTrip

Deflakes TestTransportRoundtripCloseOnWriteError.

Fixes golang/go#48995.

Change-Id: I4384d9091d55307d15fbd44b1b8137dcc8939c86
Reviewed-on: https://go-review.googlesource.com/c/net/+/356029
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

http2/transport.go

diff --git a/http2/transport.go b/http2/transport.go
index 2ff6544..9b126ea 100644
--- a/http2/transport.go
+++ b/http2/transport.go
@@ -1395,7 +1395,6 @@
 	if cs.ID != 0 {
 		cc.forgetStreamID(cs.ID)
 	}
-	close(cs.donec)
 
 	cc.wmu.Lock()
 	werr := cc.werr
@@ -1403,6 +1402,8 @@
 	if werr != nil {
 		cc.Close()
 	}
+
+	close(cs.donec)
 }
 
 // awaitOpenSlotForStream waits until len(streams) < maxConcurrentStreams.