KVM: arm64: vgic-its: Plug race in vgic_put_irq

Right now the following sequence of events can happen:

  1. Thread X calls vgic_put_irq
  2. Thread Y calls vgic_add_lpi
  3. Thread Y gets lpi_list_lock
  4. Thread X drops the ref count to 0 and blocks on lpi_list_lock
  5. Thread Y finds the irq via the lpi_list_lock, raises the ref
     count to 1, and release the lpi_list_lock.
  6. Thread X proceeds and frees the irq.

Avoid this by holding the spinlock around the kref_put.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>