mm: memcontrol: use casts when converting enums

mm: memcontrol: use casts when converting enums

Use casts when converting enum types to prevent clang warnings such
as:
  warning: implicit conversion from enumeration type 'enum node_stat_item'
  to different enumeration type 'enum memcg_stat_item' [-Wenum-conversion]

Signed-off-by: Stefan Agner <stefan@agner.ch>