mm/vmstats: add thp_split_pud event for clarity

mm/vmstats: add thp_split_pud event for clarity

We added support for PUD-sized transparent hugepages, however we count
the event "thp split pud" into thp_split_pmd event.

To separate the event count of thp split pud from pmd, add a new event
named thp_split_pud.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Yisheng Xie <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Sebastian Siewior <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Ebru Akagunduz <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Hanjun Guo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>