Fix pmap_pti_add_kva() call for doublefault stack page.

After r354889 stack got struct nmi_pcpu at top, which makes IST top
not page-aligned. Since pmap_pti_add_kva() truncates/rounds up
addresses, it erronously entered a page mapped before double fault
stack into the pti page table.

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MFC after: 3 days