staging: lustre: use ERR_CAST() function

use ERR_CAST() function instead of ERR_PTR() and PTR_ERR()
found using coccinelle and err_cast.cocci

Signed-off-by: Laurent Navet <laurent.navet@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>