staging: lustre: obdclass: constify obd_psdev_fops structure

obd_psdev_fops, of type struct file_operations, is never modified, so
declare it as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>