memcg: get rid of mem_cgroup_root_css for !CONFIG_MEMCG

The only user is cgwb_bdi_init and that one depends on
CONFIG_CGROUP_WRITEBACK which in turn depends on CONFIG_MEMCG so it
doesn't make much sense to definte an empty stub for !CONFIG_MEMCG.
Moreover ERR_PTR(-EINVAL) is ugly and would lead to runtime crashes if
used in unguarded code paths.  Better fail during compilation.
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -498,8 +498,6 @@ void mem_cgroup_split_huge_fixup(struct page *head);
 #else /* CONFIG_MEMCG */
 struct mem_cgroup;
 
-#define mem_cgroup_root_css ((struct cgroup_subsys_state *)ERR_PTR(-EINVAL))
-
 static inline void mem_cgroup_events(struct mem_cgroup *memcg,
 				     enum mem_cgroup_events_index idx,
 				     unsigned int nr)