Comment 0 for bug 1946464

Commit 5047ef5725ed ("x86/mm: Don't free P4D table when it is folded at runtime") should not be backported to v4.15 because:
1. It adds pgtable_l5_enabled which does not exist in v4.15,
2. It is marked as v4.17+
3. It is marked as fixing commit which does not exist in v4.15 and was not backported there.

allyesconfig in x86_64 fails to build:
---
In file included from ../arch/x86/include/asm/mmu_context.h:12,
                 from ../include/linux/mmu_context.h:5,
                 from ../arch/x86/kvm/../../../virt/kvm/async_pf.c:26:
../arch/x86/include/asm/pgalloc.h: In function ‘p4d_free’:
../arch/x86/include/asm/pgalloc.h:185:7: error: ‘pgtable_l5_enabled’ undeclared (first use in this function); did you mean ‘movable_node_enabled’?
  185 | if (!pgtable_l5_enabled)
      | ^~~~~~~~~~~~~~~~~~
      | movable_node_enabled

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