x86/mm: define mm_p4d_folded()

Patch series "mm/debug: Add tests validating architecture page table
helpers", v18.

This adds a test validation for architecture exported page table helpers.
Patch adds basic transformation tests at various levels of the page table.

This test was originally suggested by Catalin during arm64 THP migration
RFC discussion earlier.  Going forward it can include more specific tests
with respect to various generic MM functions like THP, HugeTLB etc and
platform specific tests.

https://lore.kernel.org/linux-mm/20190628102003.GA56463@arrakis.emea.arm.com/



This patch (of 2):

This just defines mm_p4d_folded() to check whether P4D page table level is
folded at runtime.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1587436495-22033-2-git-send-email-anshuman.khandual@arm.com
Link: http://lkml.kernel.org/r/1588564865-31160-2-git-send-email-anshuman.khandual@arm.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>