selinux: add __randomize_layout to selinux_audit_data

Randomize the layout of struct selinux_audit_data as suggested in [1],
since it contains a pointer to struct selinux_state, an already
randomized strucure.

[1]: https://github.com/KSPP/linux/issues/188



Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>