pid: use struct_size_t() helper

Before commit d67790dd ("overflow: Add struct_size_t() helper") only
struct_size() existed, which expects a valid pointer instance containing
the flexible array.

However, when we determine the default struct pid allocation size for
the associated kmem cache of a pid namespace we need to take the nesting
depth of the pid namespace into account without an variable instance
necessarily being available.

In commit b69f0aeb ("pid: Replace struct pid 1-element array with
flex-array") we used to handle this the old fashioned way and cast NULL
to a struct pid pointer type. However, we do apparently have a dedicated
struct_size_t() helper for exactly this case. So switch to that.
Suggested-by: Kees Cook <keescook@chromium.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/pid.c

@@ -657,7 +657,7 @@ void __init pid_idr_init(void)
 	idr_init(&init_pid_ns.idr);
 
 	init_pid_ns.pid_cachep = kmem_cache_create("pid",
-			struct_size((struct pid *)NULL, numbers, 1),
+			struct_size_t(struct pid, numbers, 1),
 			__alignof__(struct pid),
 			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,
 			NULL);

kernel/pid_namespace.c

@@ -48,7 +48,7 @@ static struct kmem_cache *create_pid_cachep(unsigned int level)
 		return kc;
 
 	snprintf(name, sizeof(name), "pid_%u", level + 1);
-	len = struct_size((struct pid *)NULL, numbers, level + 1);
+	len = struct_size_t(struct pid, numbers, level + 1);
 	mutex_lock(&pid_caches_mutex);
 	/* Name collision forces to do allocation under mutex. */
 	if (!*pkc)