mm/huge_memory: try avoiding write faults when changing PMD protection

Let's replicate what we have for PTEs in can_change_pte_writable() also
for PMDs.

While this might look like a pure performance improvement, we'll us this to
get rid of savedwrite handling in do_huge_pmd_numa_page() next. Place
do_huge_pmd_numa_page() strategically good for that purpose.

Note that MM_CP_TRY_CHANGE_WRITABLE is currently only set when we come
via mprotect_fixup().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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