selinux: Use struct_size() helper in kmalloc()

Make use of struct_size() helper instead of an open-coded calculation.

Link: https://github.com/KSPP/linux/issues/160


Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>