This is clearer than syscall.EBADF.

Fixes #17320.

Change-Id: I14c6a362f9a6044c9b07cd7965499f4a83d2a860
Reviewed-on: https://go-review.googlesource.com/30614
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>