cgroup: Avoid compiler warnings with no subsystems

As done before in commit cb4a3167

 ("cgroup: use bitmask to filter
for_each_subsys"), avoid compiler warnings for the pathological case of
having no subsystems (i.e. CGROUP_SUBSYS_COUNT == 0). This condition is
hit for the arm multi_v7_defconfig config under -Wzero-length-bounds:

In file included from ./arch/arm/include/generated/asm/rwonce.h:1,
                 from include/linux/compiler.h:264,
                 from include/uapi/linux/swab.h:6,
                 from include/linux/swab.h:5,
                 from arch/arm/include/asm/opcodes.h:86,
                 from arch/arm/include/asm/bug.h:7,
                 from include/linux/bug.h:5,
                 from include/linux/thread_info.h:13,
                 from include/asm-generic/current.h:5,
                 from ./arch/arm/include/generated/asm/current.h:1,
                 from include/linux/sched.h:12,
                 from include/linux/cgroup.h:12,
                 from kernel/cgroup/cgroup-internal.h:5,
                 from kernel/cgroup/cgroup.c:31:
kernel/cgroup/cgroup.c: In function 'of_css':
kernel/cgroup/cgroup.c:651:42: warning: array subscript '<unknown>' is outside the bounds of an
interior zero-length array 'struct cgroup_subsys_state *[0]' [-Wzero-length-bounds]
  651 |   return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: cgroups@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tejun Heo <tj@kernel.org>