Like on linux, lsetxattr on symlinks returns EPERM.

Change-Id: I6dfc59484e1826872af54579dd6ec02705c816da
Reviewed-on: https://go-review.googlesource.com/114535
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>