irqchip/gic-v3-its: Synchronise INV command targetting a VLPI using VSYNC

We have so far alwways invalidated VLPIs usinc an INV+SYNC
sequence, but that's pretty wrong for two reasons:

- SYNC only synchronises physical LPIs
- The collection ID that for the associated LPI doesn't match
  the redistributor the vPE is associated with

Instead, send an INV+VSYNC for forwarded LPIs, ensuring that
the ITS can properly synchronise the invalidation of VLPIs.

Fixes: 015ec038 ("irqchip/gic-v3-its: Add VLPI configuration handling")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191108165805.3071-9-maz@kernel.org