cgroup: Eliminate cgrp_ancestor_storage in cgroup_root

The cgrp_ancestor_storage has two drawbacks:
- it's not guaranteed that the member immediately follows struct cgrp in
cgroup_root (root cgroup's ancestors[0] might thus point to a padding
and not in cgrp_ancestor_storage proper),
- this idiom raises warnings with -Wflex-array-member-not-at-end.

Instead of relying on the auxiliary member in cgroup_root, define the
0-th level ancestor inside struct cgroup (needed for static allocation
of cgrp_dfl_root), deeper cgroups would allocate flexible
_low_ancestors[]. Unionized alias through ancestors[] will
transparently join the two ranges.

The above change would still leave the flexible array at the end of
struct cgroup inside cgroup_root, so move cgrp also towards the end of
cgroup_root to resolve the -Wflex-array-member-not-at-end.

Link: https://lore.kernel.org/r/[e-mail]/
Reported-by: Gustavo A. R. Silva <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Cc: David Laight <[e-mail]>
Acked-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Michal Koutný <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

include/linux/cgroup-defs.h

···
 #endif
 
 	/* All ancestors including self */
-	struct cgroup *ancestors[];
+	union {
+		DECLARE_FLEX_ARRAY(struct cgroup *, ancestors);
+		struct {
+			struct cgroup *_root_ancestor;
+			DECLARE_FLEX_ARRAY(struct cgroup *, _low_ancestors);
+		};
+	};
 };
 
 /*
···
 	struct list_head root_list;
 	struct rcu_head rcu;	/* Must be near the top */
 
-	/*
-	 * The root cgroup. The containing cgroup_root will be destroyed on its
-	 * release. cgrp->ancestors[0] will be used overflowing into the
-	 * following field. cgrp_ancestor_storage must immediately follow.
-	 */
-	struct cgroup cgrp;
-
-	/* must follow cgrp for cgrp->ancestors[0], see above */
-	struct cgroup *cgrp_ancestor_storage;
-
 	/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
 	atomic_t nr_cgrps;
 
···
 
 	/* The name for this hierarchy - may be empty */
 	char name[MAX_CGROUP_ROOT_NAMELEN];
+
+	/*
+	 * The root cgroup. The containing cgroup_root will be destroyed on its
+	 * release. This must be embedded last due to flexible array at the end
+	 * of struct cgroup.
+	 */
+	struct cgroup cgrp;
 };
 
 /*

kernel/cgroup/cgroup.c

···
 	int ret;
 
 	/* allocate the cgroup and its ID, 0 is reserved for the root */
-	cgrp = kzalloc(struct_size(cgrp, ancestors, (level + 1)), GFP_KERNEL);
+	cgrp = kzalloc(struct_size(cgrp, _low_ancestors, level), GFP_KERNEL);
 	if (!cgrp)
 		return ERR_PTR(-ENOMEM);