net/http: fix server/transport data race when sharing the request body

Introduced in https://go-review.googlesource.com/12865 (git rev c2db5f4c).

This fix doesn't add any new lock acquistions: it just moves the
existing one taken by the unreadDataSize method and moves it out
wider.

It became flaky at rev c2db5f4c, but now reliably passes again:
$ go test -v -race -run=TestTransportAndServerSharedBodyRace -count=100 net/http

Fixes #11985

Change-Id: I6956d62839fd7c37e2f7441b1d425793f4a0db30
Reviewed-on: https://go-review.googlesource.com/12909
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/server.go

@@ -880,17 +880,19 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 				discard = true
 			}
 		case *body:
+			bdy.mu.Lock()
 			switch {
 			case bdy.closed:
 				if !bdy.sawEOF {
 					// Body was closed in handler with non-EOF error.
 					w.closeAfterReply = true
 				}
-			case bdy.unreadDataSize() >= maxPostHandlerReadBytes:
+			case bdy.unreadDataSizeLocked() >= maxPostHandlerReadBytes:
 				tooBig = true
 			default:
 				discard = true
 			}
+			bdy.mu.Unlock()
 		default:
 			discard = true
 		}