mm: thp: enable thp migration in generic path

mm: thp: enable thp migration in generic path

Add thp migration's core code, including conversions between a PMD entry
and a swap entry, setting PMD migration entry, removing PMD migration
entry, and waiting on PMD migration entries.

This patch makes it possible to support thp migration.  If you fail to
allocate a destination page as a thp, you just split the source thp as
we do now, and then enter the normal page migration.  If you succeed to
allocate destination thp, you enter thp migration.  Subsequent patches
actually enable thp migration for each caller of page migration by
allowing its get_new_page() callback to allocate thps.

[[email protected]: fix gcc-4.9.0 -Wmissing-braces warning]
Link: http://lkml.kernel.org/r/[email protected]
[[email protected]: fix x86_64 allnoconfig warning]
Signed-off-by: Zi Yan <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Nellans <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>