KVM: arm/arm64: Fix userspace_irqchip_in_use counting

We were not decrementing the static key count in the right location.
kvm_arch_vcpu_destroy() is only called to clean up after a failed
VCPU create attempt, whereas kvm_arch_vcpu_free() is called on teardown
of the VM as well.  Move the static key decrement call to
kvm_arch_vcpu_free().

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>