KVM: arm64: vgic: Ensure that slots_lock is held in vgic_register_all_redist_iodevs()

Although we implicitly depend on slots_lock being held when registering
IO devices with the IO bus infrastructure, we don't enforce this
requirement. Make it explicit.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231207151201.3028710-5-maz@kernel.org


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>