vfio/pci: call irq_bypass_unregister_producer() before freeing irq

vfio/pci: call irq_bypass_unregister_producer() before freeing irq

Since irq_bypass_register_producer() is called after request_irq(), we
should do tear-down in reverse order: irq_bypass_unregister_producer()
then free_irq().

Specifically free_irq() may release resources required by the
irqbypass del_producer() callback.  Notably an example provided by
Marc Zyngier on arm64 with GICv4 that he indicates has the potential
to wedge the hardware:

 free_irq(irq)
   __free_irq(irq)
     irq_domain_deactivate_irq(irq)
       its_irq_domain_deactivate()
         [unmap the VLPI from the ITS]

 kvm_arch_irq_bypass_del_producer(cons, prod)
   kvm_vgic_v4_unset_forwarding(kvm, irq, ...)
     its_unmap_vlpi(irq)
       [Unmap the VLPI from the ITS (again), remap the original LPI]

Signed-off-by: Jiang Yi <giangyi@amazon.com>
Cc: stable@vger.kernel.org # v4.4+
Fixes: 6d7425f109d26 ("vfio: Register/unregister irq_bypass_producer")
Link: https://lore.kernel.org/kvm/20191127164910.15888-1-giangyi@amazon.com
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
[aw: commit log]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>