net: defer file.close() + minor style cleanup

net: defer file.close() + minor style cleanup

Moved the relevant file.close() usages close to after the
file opens and put them in defer statements, so that readers
don't have to think too much as to where the file is
being closed.

Change-Id: Ic4190b02ea2f5ac281b9ba104e0023e9f87ca8c7
Reviewed-on: https://go-review.googlesource.com/41796
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>