mm/debug_vm_pgtable: fix BUG_ON with pud advanced test

In the Linux kernel, the following vulnerability has been resolved:

mm/debug_vm_pgtable: fix BUG_ON with pud advanced test

Architectures like powerpc add debug checks to ensure we find only devmap

PUD pte entries. These debug checks are only done with CONFIG_DEBUG_VM.

This patch marks the ptes used for PUD advanced test devmap pte entries so

that we don't hit on debug checks on architecture like ppc64 as below.

WARNING: CPU: 2 PID: 1 at arch/powerpc/mm/book3s64/radix_pgtable.c:1382 radix__pud_hugepage_update+0x38/0x138

....

NIP [c0000000000a7004] radix__pud_hugepage_update+0x38/0x138

LR [c0000000000a77a8] radix__pudp_huge_get_and_clear+0x28/0x60

Call Trace:

[c000000004a2f950] [c000000004a2f9a0] 0xc000000004a2f9a0 (unreliable)

[c000000004a2f980] [000d34c100000000] 0xd34c100000000

[c000000004a2f9a0] [c00000000206ba98] pud_advanced_tests+0x118/0x334

[c000000004a2fa40] [c00000000206db34] debug_vm_pgtable+0xcbc/0x1c48

[c000000004a2fc10] [c00000000000fd28] do_one_initcall+0x60/0x388

Also

kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:202!

....

NIP [c000000000096510] pudp_huge_get_and_clear_full+0x98/0x174

LR [c00000000206bb34] pud_advanced_tests+0x1b4/0x334

Call Trace:

[c000000004a2f950] [000d34c100000000] 0xd34c100000000 (unreliable)

[c000000004a2f9a0] [c00000000206bb34] pud_advanced_tests+0x1b4/0x334

[c000000004a2fa40] [c00000000206db34] debug_vm_pgtable+0xcbc/0x1c48

[c000000004a2fc10] [c00000000000fd28] do_one_initcall+0x60/0x388