arch/x86_64: fix revoke_low_memory

[xianglyc]

the virtual address of the page table (PT) is saved into the page directory (PD) entry, but the PD entry should store the physical address instead

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Summary

the virtual address of the page table (PT) is saved into the page directory (PD) entry, but the PD entry should store the physical address instead

Update this section with information on why change is necessary,
what it exactly does and how, if new feature shows up, provide
references (dependencies, similar problems and solutions), etc.

Impact

no
Update this section, where applicable, on how change affects users,
build process, hardware, documentation, security, compatibility, etc.

Testing

ostest
This section should provide a detailed description of what you did
to verify your changes work and do not break existing code.

Please provide information about your host machine, the board(s) you
tested your changes on, and how you tested. Logs should be included.

For example, when changing something in the core OS functions, you
may want to run the OSTest application to verify that there are no
regressions. Changes to ADC code may warrant running the adc
example. Adding a new uORB driver may require that you run
uorb_listener to verify correct operation.

Pure documentation changes can just be tested with make html
(see docs) and verification of the correct format in your
browser.

PRs without testing information will not be accepted. We will
request test logs.