mm: vmstat: get slab statistics always from node counters

mm: vmstat: get slab statistics always from node counters

After the move of slab statistics from zone to node counters some
users still try to get the counters from the zone counters. This has
been caught while compiling with clang printing a warning like:

  implicit conversion from enumeration type 'enum node_stat_item' to
  different enumeration type 'enum zone_stat_item' [-Wenum-conversion]

Fixes: 385386cff4 ("mm: vmstat: move slab statistics from zone to node counters")
Signed-off-by: Stefan Agner <stefan@agner.ch>