mm/vmpressure.c: fix subtree pressure detection

mm/vmpressure.c: fix subtree pressure detection

When vmpressure is called for the entire subtree under pressure we
mistakenly use vmpressure->scanned instead of vmpressure->tree_scanned
when checking if vmpressure work is to be scheduled.  This results in
suppressing all vmpressure events in the legacy cgroup hierarchy.  Fix it.

Fixes: 8e8ae645249b ("mm: memcontrol: hook up vmpressure to socket pressure")
Signed-off-by: Vladimir Davydov <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>