Fsetxattr should just return an error (like Setxattr and like Fsetxattr
is already doing on FreeBSD).

Fix for CL 128176

Updates golang/go#26832

Change-Id: Id1c1315d9eb6952128e0c0ce44b19060e0403cc0
Reviewed-on: https://go-review.googlesource.com/128495
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>