pid: Replace struct pid 1-element array with flex-array

For pid namespaces, struct pid uses a dynamically sized array member,
"numbers". This was implemented using the ancient 1-element fake
flexible array, which has been deprecated for decades.

Replace it with a C99 flexible array, refactor the array size
calculations to use struct_size(), and address elements via indexes.
Note that the static initializer (which defines a single element) works
as-is, and requires no special handling.

Without this, CONFIG_UBSAN_BOUNDS (and potentially
CONFIG_FORTIFY_SOURCE) will trigger bounds checks:

https://lore.kernel.org/lkml/20230517-bushaltestelle-super-e223978c1ba6@brauner

Cc: Christian Brauner <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Jeff Xu <[e-mail]>
Cc: Andreas Gruenbacher <[e-mail]>
Cc: Daniel Verkamp <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Jeff Xu <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Luis Chamberlain <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Reported-by: [e-mail]
[brauner: dropped unrelated changes and remove 0 with NULL cast]
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/pid.h

···
 	/* wait queue for pidfd notifications */
 	wait_queue_head_t wait_pidfd;
 	struct rcu_head rcu;
-	struct upid numbers[1];
+	struct upid numbers[];
 };
 
 extern struct pid init_struct_pid;

kernel/pid.c

···
 
 	idr_init(&init_pid_ns.idr);
 
-	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
-			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
+	init_pid_ns.pid_cachep = kmem_cache_create("pid",
+			struct_size((struct pid *)NULL, numbers, 1),
+			__alignof__(struct pid),
+			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,
+			NULL);
 }
 
 static struct file *__pidfd_fget(struct task_struct *task, int fd)

kernel/pid_namespace.c

···
 		return kc;
 
 	snprintf(name, sizeof(name), "pid_%u", level + 1);
-	len = sizeof(struct pid) + level * sizeof(struct upid);
+	len = struct_size((struct pid *)NULL, numbers, level + 1);
 	mutex_lock(&pid_caches_mutex);
 	/* Name collision forces to do allocation under mutex. */
 	if (!*pkc)