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>