Our LPI translation cache needs to be able to drop the refcount
on an LPI whilst already holding the lpi_list_lock.

Let's add a new primitive for this.
Tested-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>