We are about to distinguish between userspace accesses and mmio traps
for a number of the mmio handlers.  When the requester vcpu is NULL, it
means we are handling a userspace access.

Factor out the functionality to get the request vcpu into its own
function, mostly so we have a common place to document the semantics of
the return value.

Also take the chance to move the functionality outside of holding a
spinlock and instead explicitly disable and enable preemption.  This
supports PREEMPT_RT kernels as well.
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>