KVM: Nullify async #PF worker's "apf" pointer as soon as it might be freed

Nullify the async #PF worker's local "apf" pointer immediately after the
point where the structure can be freed by the vCPU.  The existing comment
is helpful, but easy to overlook as there is no associated code.

Update the comment to clarify that it can be freed by as soon as the lock
is dropped, as "after this point" isn't strictly accurate, nor does it
help understand what prevents the structure from being freed earlier.
Reviewed-by: Xu Yilun <yilun.xu@intel.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240110011533.503302-5-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/async_pf.c

@@ -83,13 +83,14 @@ static void async_pf_execute(struct work_struct *work)
 	apf->vcpu = NULL;
 	spin_unlock(&vcpu->async_pf.lock);
 
-	if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
-		kvm_arch_async_page_present_queued(vcpu);
-
 	/*
-	 * apf may be freed by kvm_check_async_pf_completion() after
-	 * this point
+	 * The apf struct may be freed by kvm_check_async_pf_completion() as
+	 * soon as the lock is dropped.  Nullify it to prevent improper usage.
 	 */
+	apf = NULL;
+
+	if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
+		kvm_arch_async_page_present_queued(vcpu);
 
 	trace_kvm_async_pf_completed(addr, cr2_or_gpa);