path/filepath: fix evaluation of symlinks to paths under /tmp on macOS

path/filepath: fix evaluation of symlinks to paths under /tmp on macOS

For symlinks created from symlinks under the root directory created
as the relative path (e.g., symbolic links under /tmp), we update vol and volLen.

Fixes #57905

Change-Id: I45affd1db3b93109de51bf19b181f3cdba061109
Reviewed-on: https://go-review.googlesource.com/c/go/+/461761
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>