It's only ever called with the value it was using, but the code was
counterintuitive. Use the parameter instead, like the other funcs near
it.

Found by github.com/mvdan/unparam.

Change-Id: I45855e11d749380b9b2a28e6dd1d5dedf119a19b
Reviewed-on: https://go-review.googlesource.com/37893Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>