http2: fix race on ClientConn.maxFrameSize
http2: fix race on ClientConn.maxFrameSize

This fixes TestTrailersClientToServer_h2. Before this CL, the following
command reliably fails. With this CL merged into net/http, the following
command reliably succeeds.

go test -race -run=TestTrailersClientToServer_h2 -count 1000 net/http

Updates golang/go#22721

Change-Id: I05d1504c60854fcf3ae9531f36a126e94b00f0b7
Reviewed-on: https://go-review.googlesource.com/79238
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

golang/net