irqchip/gic-v3-its: Fix command buffer allocation

The its command buffer must be page aligned, but kzalloc() is not
guaranteed to be (though it is mostly when allocating 64k). Use
__get_free_pages() as this is used for other buffers as well.

Signed-off-by: Robert Richter <rrichter@cavium.com>
[Marc: fixed the error path]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>