mm/hmm/mirror: helper to snapshot CPU page table

mm/hmm/mirror: helper to snapshot CPU page table

This does not use existing page table walker because we want to share
same code for our page fault handler.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jérôme Glisse <[email protected]>
Signed-off-by: Evgeny Baskakov <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Mark Hairgrove <[email protected]>
Signed-off-by: Sherry Cheung <[email protected]>
Signed-off-by: Subhash Gutti <[email protected]>
Cc: Aneesh Kumar <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: David Nellans <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Ross Zwisler <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Bob Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>