mm: workingset: add vmstat counter for shadow nodes

mm: workingset: add vmstat counter for shadow nodes

Make it easier to catch bugs in the shadow node shrinker by adding a
counter for the shadow nodes in circulation.

[[email protected]: assert that irqs are disabled, for __inc_lruvec_page_state()]
[[email protected]: s/WARN_ON_ONCE/VM_WARN_ON_ONCE/, per Johannes]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Johannes Weiner <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>