KVM: Assert that struct kvm_vcpu is always as offset zero

KVM implementations that wrap struct kvm_vcpu with a vendor specific
struct, e.g. struct vcpu_vmx, must place the vcpu member at offset 0,
otherwise the usercopy region intended to encompass struct kvm_vcpu_arch
will instead overlap random chunks of the vendor specific struct.
E.g. padding a large number of bytes before struct kvm_vcpu triggers
a usercopy warn when running with CONFIG_HARDENED_USERCOPY=y.

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

arch/x86/kvm/svm.c

@@ -2133,6 +2133,9 @@ static struct kvm_vcpu *svm_create_vcpu(struct kvm *kvm, unsigned int id)
 	struct page *nested_msrpm_pages;
 	int err;
 
+	BUILD_BUG_ON_MSG(offsetof(struct vcpu_svm, vcpu) != 0,
+		"struct kvm_vcpu must be at offset 0 for arch usercopy region");
+
 	svm = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL_ACCOUNT);
 	if (!svm) {
 		err = -ENOMEM;