staging/lustre: Replace sun.com GPLv2 URL with gnu.org one.

http://www.sun.com/software/products/lustre/docs/GPLv2.pdf is no
longer around, so replae it with (hopefully more permanent)
http://http://www.gnu.org/licenses/gpl-2.0.html



Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reported-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>