KVM: PPC: e500mc: Add build-time assert that vcpu is at offset 0

In preparation for moving vcpu allocation to common PPC code, add an
explicit, albeit redundant, build-time assert to ensure the vcpu member
is located at offset 0.  The assert is redundant in the sense that
kvmppc_core_vcpu_create_e500() contains a functionally identical assert.
The motiviation for adding the extra assert is to provide visual
confirmation of the correctness of moving vcpu allocation to common
code.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/powerpc/kvm/e500mc.c

@@ -308,6 +308,8 @@ static struct kvm_vcpu *kvmppc_core_vcpu_create_e500mc(struct kvm *kvm,
 	struct kvm_vcpu *vcpu;
 	int err;
 
+	BUILD_BUG_ON(offsetof(struct kvmppc_vcpu_e500, vcpu) != 0);
+
 	vcpu_e500 = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL);
 	if (!vcpu_e500) {
 		err = -ENOMEM;