CVE-2026-72283: KVM: x86: Nullify irqfd->producer if updating IRTE for bypass fails

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Nullify irqfd->producer if updating IRTE for bypass fails

Nullify irqfd->producer if updating the IRTE for bypass fails, as leaving a dangling pointer will result in a use-after-free if the irqfd is reachable through KVM's routing, but the producer is freed separately. E.g. for VFIO PCI, the producer is embedded in struct "vfiopciirqctx" and freed when the vector is disabled, which can happen independent of routing updates.

[sean: drop PPC change, massage changelog]