KVM: Fix last_boosted_vcpu index assignment bug

In kvm_vcpu_on_spin(), the loop counter 'i' is incorrectly written to
last_boosted_vcpu instead of the actual vCPU index 'idx'. This causes
last_boosted_vcpu to store the loop iteration count rather than the
vCPU index, leading to incorrect round-robin behavior in subsequent
directed yield operations.

Fix this by using 'idx' instead of 'i' in the assignment.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20251110033232.12538-7-kernellwp@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Wanpeng Li

virt/kvm/kvm_main.c

@@ -4026,7 +4026,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 
 		yielded = kvm_vcpu_yield_to(vcpu);
 		if (yielded > 0) {
-			WRITE_ONCE(kvm->last_boosted_vcpu, i);
+			WRITE_ONCE(kvm->last_boosted_vcpu, idx);
 			break;
 		} else if (yielded < 0 && !--try) {
 			break;