path/filepath: walkSymlinks: return correct error for file with trailing slash

Rather than return os.ErrNotExist for /path/to/existing_file/,
walkSymLinks now returns syscall.ENOTDIR.

This is consistent with behavior of os.Lstat.

Fixes #29372

Change-Id: Id5c471d901db04b2f35d60f60a81b2a0be93cae9
Reviewed-on: https://go-review.googlesource.com/c/155597


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>