KVM: arm64: Protect vLPI translation with vgic_irq::irq_lock

KVM: arm64: Protect vLPI translation with vgic_irq::irq_lock

Though undocumented, KVM generally protects the translation of a vLPI
with the its_lock. While this makes perfectly good sense, as the ITS
itself contains the guest translation, an upcoming change will require
twiddling the vLPI mapping in an atomic context.

Switch to using the vIRQ's irq_lock to protect the translation. Use of
the its_lock in vgic_v4_unset_forwarding() is preserved for now as it
still needs to walk the ITS.

Tested-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250523194722.4066715-3-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>