KVM: arm64: vgic: Allow registration of a non-maskable maintenance interrupt

Our Apple M1/M2 friends do have a per-CPU maintenance interrupt,
but no mask to make use of it in the standard Linux framework.

Given that KVM directly drives the *source* of the interrupt and
leaves the GIC interrupt always enabled, there is no harm in tolerating
such a setup. It will become useful once we enable NV on M2 HW.

Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Oliver Upton <[e-mail]>

arch/arm64/kvm/vgic/vgic-init.c

···
 	if (ret)
 		return ret;
 
-	if (!has_mask)
+	if (!has_mask && !kvm_vgic_global_state.maint_irq)
 		return 0;
 
 	ret = request_percpu_irq(kvm_vgic_global_state.maint_irq,