mm: add per-order mTHP swpin counters

This helps profile the sizes of folios being swapped in. Currently,
only mTHP swap-out is being counted.
The new interface can be found at:
/sys/kernel/mm/transparent_hugepage/hugepages-<size>/stats
         swpin
For example,
cat /sys/kernel/mm/transparent_hugepage/hugepages-64kB/stats/swpin
12809
cat /sys/kernel/mm/transparent_hugepage/hugepages-32kB/stats/swpin
4763

[[email protected]: add a blank line in doc]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Barry Song <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Chris Li <[email protected]>
Cc: Yosry Ahmed <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Kanchana P Sridhar <[email protected]>
Cc: Usama Arif <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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