KVM: Don't BUG() the kernel if xa_insert() fails with -EBUSY

WARN once instead of triggering a BUG if xa_insert() fails because it
encountered an existing entry.  While KVM guarantees there should be no
existing entry, there's no reason to BUG the kernel, as KVM needs to
gracefully handle failure anyways.

Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241009150455.1057573-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

virt/kvm/kvm_main.c

@@ -4129,7 +4129,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
 
 	vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
 	r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
-	BUG_ON(r == -EBUSY);
+	WARN_ON_ONCE(r == -EBUSY);
 	if (r)
 		goto unlock_vcpu_destroy;