x86_64/HOTPLUG_CPU: NULL dereference doesn't #PF with init_level4_pgt

NULL dereferences don't cause a page fault if the 4th level pagetable 
being used is init_level4_pgt because we never zap_low_mappings. Since 
the idle thread uses init_level4_pgt any bad dereferences happening there 
(e.g. from interrupts) won't cause a fault. Andi would you be fine with 
switching the idle threads to a different level4?

Thanks,
	Zwane

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