irqchip/gic-v3-its: Lock VLPI map array before translating it

Obtaining the mapping ivformation for a VLPI should always be
done with the vlpi_lock for this device held. Otherwise, we
expose ourselves to races against a concurrent unmap.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191108165805.3071-11-maz@kernel.org