kvm: selftests: aarch64: use a tighter assert in vgic_poke_irq()

vgic_poke_irq() checks that the attr argument passed to the vgic device
ioctl is sane. Make this check tighter by moving it to after the last
attr update.

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reported-by: Reiji Watanabe <reijiw@google.com>
Cc: Andrew Jones <drjones@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220127030858.3269036-6-ricarkol@google.com