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/20240726150728.3159964-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>