Fix index calculation for the page table pages for efirt 1:1 map.

[kib]

Summary

Stop issuing pre-assigned number to enumerate all page table pages, instead automatically calculate next free index. This index in fact does not serve any purpose except to be unique to satisfy vm_page_grab() interface. We do not look up the page by the index later.

Reported by: emaste
PR: 223906

Test Plan

I did not tested it, only compiled on amd64. It requires check both on amd64 and arm64.

[emaste]

EFI RTC works on 96-core arm64 ThunderX with this change.