[internal-branch.go1.16-vendor] http2: fix race in DATA frame padding…

… refund

Move flow adjustment back under cc.mu.

Updates golang/go#49076

Change-Id: Idb762091cfeb55c18bc74389e62193f81438624f
Reviewed-on: https://go-review.googlesource.com/c/net/+/351950
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/net/+/356983
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

http2/transport.go

@@ -2338,14 +2338,16 @@ func (rl *clientConnReadLoop) processData(f *DataFrame) error {
 		}
 		if refund > 0 {
 			cc.inflow.add(int32(refund))
+			if !didReset {
+				cs.inflow.add(int32(refund))
+			}
 		}
 		cc.mu.Unlock()
 
 		if refund > 0 {
 			cc.wmu.Lock()
 			cc.fr.WriteWindowUpdate(0, uint32(refund))
 			if !didReset {
-				cs.inflow.add(int32(refund))
 				cc.fr.WriteWindowUpdate(cs.ID, uint32(refund))
 			}
 			cc.bw.Flush()