KVM: arm64: Use lock guard in vgic_v4_set_forwarding()

The locking dance is about to get more interesting, switch the its_lock
over to a lock guard to make it a bit easier to handle.

Tested-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250523194722.4066715-2-oliver.upton@linux.dev


Signed-off-by: Marc Zyngier <maz@kernel.org>