Updates http2 to x/net git rev 894f8ed58 for:

    http2: fix flake in net/http's TestCloseIdleConnections_h2
    https://golang.org/cl/80139

    http2: fix leak in activeRes by removing activeRes
    https://golang.org/cl/80137

Fixes #22413
Fixes #21543

Change-Id: Ic8ea20f8ddae2fde17884ed045f9fa7058a4bd23
Reviewed-on: https://go-review.googlesource.com/81276
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>