mm/debug_vm_pgtable: drop RANDOM_ORVALUE trick

Macro RANDOM_ORVALUE was used to make sure the pgtable entry will be
populated with !none data in clear tests.

The RANDOM_ORVALUE tried to cover mostly all the bits in a pgtable entry,
even if there's no discussion on whether all the bits will be vaild.  Both
S390 and PPC64 have their own masks to avoid touching some bits.  Now it's
the turn for x86_64.

The issue is there's a recent report from Mikhail Gavrilov showing that
this can cause a warning with the newly added pte set check in commit
8430557fc5 on writable v.s.  userfaultfd-wp bit, even though the check
itself was valid, the random pte is not.  We can choose to mask more bits
out.

However the need to have such random bits setup is questionable, as now
it's already guaranteed to be true on below:

  - For pte level, the pgtable entry will be installed with value from
    pfn_pte(), where pfn points to a valid page.  Hence the pte will be
    !none already if populated with pfn_pte().

  - For upper-than-pte level, the pgtable entry should contain a directory
    entry always, which is also !none.

All the cases look like good enough to test a pxx_clear() helper.  Instead
of extending the bitmask, drop the "set random bits" trick completely.  Add
some warning guards to make sure the entries will be !none before clear().

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 8430557fc584 ("mm/page_table_check: support userfault wr-protect entries")
Signed-off-by: Peter Xu <[email protected]>
Reported-by: Mikhail Gavrilov <[email protected]>
Link: https://lore.kernel.org/r/CABXGCsMB9A8-X+Np_Q+fWLURYL_0t3Y-MdoNabDM-Lzk58-DGA@mail.gmail.com
Tested-by: Mikhail Gavrilov <[email protected]>
Reviewed-by: Pasha Tatashin <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Gavin Shan <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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