powerpc/8xx: document and enforce that split PT locks are not used

Right now, we cannot have split PT locks because 8xx does not support SMP.

But for the sake of documentation *why* 8xx is fine regarding what we
documented in huge_pte_lockptr(), let's just add code to enforce it at the
same time as documenting it.

This should also make everybody who wants to copy from the 8xx approach of
supporting such unusual ways of mapping hugetlb folios aware that it gets
tricky once multiple page tables are involved.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Rapoport (Microsoft) <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: "Naveen N. Rao" <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Russell King <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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