If the walker function called on a top-level file returns SkipDir,
then (before this change) Walk would return SkipDir, which the
documentation implies will not happen.

Fixes #16280.

Change-Id: I37d63bdcef7af4b56e342b624cf0d4b42e65c297
Reviewed-on: https://go-review.googlesource.com/24780
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>