irqchip/gic-v4: Don't allow a VMOVP on a dying VPE

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

irqchip/gic-v4: Don't allow a VMOVP on a dying VPE

Kunkun Jiang reported that there is a small window of opportunity for

userspace to force a change of affinity for a VPE while the VPE has already

been unmapped, but the corresponding doorbell interrupt still visible in

/proc/irq/.

Plug the race by checking the value of vmapp_count, which tracks whether

the VPE is mapped ot not, and returning an error in this case.

This involves making vmapp_count common to both GICv4.1 and its v4.0

ancestor.