KVM: arm64: vgic: Fix IIDR revision field extracted from wrong value

KVM: arm64: vgic: Fix IIDR revision field extracted from wrong value

The uaccess write handlers for GICD_IIDR in both GICv2 and GICv3
extract the revision field from 'reg' (the current IIDR value read back
from the emulated distributor) instead of 'val' (the value userspace is
trying to write). This means userspace can never actually change the
implementation revision — the extracted value is always the current one.

Fix the FIELD_GET to use 'val' so that userspace can select a different
revision for migration compatibility.

Fixes: 49a1a2c70a7f ("KVM: arm64: vgic-v3: Advertise GICR_CTLR.{IR, CES} as a new GICD_IIDR revision")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://patch.msgid.link/20260407210949.2076251-2-dwmw2@infradead.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org