KVM: arm64: Fix __pkvm_init_vcpu cptr_el2 error path

On an error, hyp_vcpu will be accessed while this memory has already
been relinquished to the host and unmapped from the hypervisor. Protect
the CPTR assignment with an early return.

Fixes: b5b99556 ("KVM: arm64: Eagerly restore host fpsimd/sve state in pKVM")
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Link: https://lore.kernel.org/r/20240919110500.2345927-1-vdonnefort@google.comSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/hyp/nvhe/pkvm.c

@@ -574,12 +574,14 @@ int __pkvm_init_vcpu(pkvm_handle_t handle, struct kvm_vcpu *host_vcpu,
 unlock:
 	hyp_spin_unlock(&vm_table_lock);
 
-	if (ret)
+	if (ret) {
 		unmap_donated_memory(hyp_vcpu, sizeof(*hyp_vcpu));
+		return ret;
+	}
 
 	hyp_vcpu->vcpu.arch.cptr_el2 = kvm_get_reset_cptr_el2(&hyp_vcpu->vcpu);
 
-	return ret;
+	return 0;
 }
 
 static void