mm/powerpc: add missing pud helpers

Some new helpers will be needed for pud entry updates soon.  Introduce
these helpers by referencing the pmd ones.  Namely:

  - pudp_invalidate(): this helper invalidates a huge pud before a split
  happens, so that the invalidated pud entry will make sure no race will
  happen (either with software, like a concurrent zap, or hardware, like
  a/d bit lost).

  - pud_modify(): this helper applies a new pgprot to an existing huge pud
  mapping.

For more information on why we need these two helpers, please refer to the
corresponding pmd helpers in the mprotect() code path.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: "Edgecombe, Rick P" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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