xen/arm: take the rank lock before accessing ipriority

xen/arm: take the rank lock before accessing ipriority

Currently we read ipriority from vgic_vcpu_inject_irq without taking the
rank lock. Fix that by taking the rank lock and reading ipriority at the
beginning of the function.

As vgic_vcpu_inject_irq is called from the irq.c upon receiving an
interrupt, we need to change the implementation of vgic_lock/unlock_rank
to spin_lock_irqsave to make it safe in irq context.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>