bpf: fix compilation error without CGROUPS

Our MPTCP CI complained [1] -- and KBuild too -- that it was no longer
possible to build the kernel without CONFIG_CGROUPS:

  kernel/bpf/task_iter.c: In function 'bpf_iter_css_task_new':
  kernel/bpf/task_iter.c:919:14: error: 'CSS_TASK_ITER_PROCS' undeclared (first use in this function)
    919 |         case CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED:
        |              ^~~~~~~~~~~~~~~~~~~
  kernel/bpf/task_iter.c:919:14: note: each undeclared identifier is reported only once for each function it appears in
  kernel/bpf/task_iter.c:919:36: error: 'CSS_TASK_ITER_THREADED' undeclared (first use in this function)
    919 |         case CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED:
        |                                    ^~~~~~~~~~~~~~~~~~~~~~
  kernel/bpf/task_iter.c:927:60: error: invalid application of 'sizeof' to incomplete type 'struct css_task_iter'
    927 |         kit->css_it = bpf_mem_alloc(&bpf_global_ma, sizeof(struct css_task_iter));
        |                                                            ^~~~~~
  kernel/bpf/task_iter.c:930:9: error: implicit declaration of function 'css_task_iter_start'; did you mean 'task_seq_start'? [-Werror=implicit-function-declaration]
    930 |         css_task_iter_start(css, flags, kit->css_it);
        |         ^~~~~~~~~~~~~~~~~~~
        |         task_seq_start
  kernel/bpf/task_iter.c: In function 'bpf_iter_css_task_next':
  kernel/bpf/task_iter.c:940:16: error: implicit declaration of function 'css_task_iter_next'; did you mean 'class_dev_iter_next'? [-Werror=implicit-function-declaration]
    940 |         return css_task_iter_next(kit->css_it);
        |                ^~~~~~~~~~~~~~~~~~
        |                class_dev_iter_next
  kernel/bpf/task_iter.c:940:16: error: returning 'int' from a function with return type 'struct task_struct *' makes pointer from integer without a cast [-Werror=int-conversion]
    940 |         return css_task_iter_next(kit->css_it);
        |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  kernel/bpf/task_iter.c: In function 'bpf_iter_css_task_destroy':
  kernel/bpf/task_iter.c:949:9: error: implicit declaration of function 'css_task_iter_end' [-Werror=implicit-function-declaration]
    949 |         css_task_iter_end(kit->css_it);
        |         ^~~~~~~~~~~~~~~~~

This patch simply surrounds with a #ifdef the new code requiring CGroups
support. It seems enough for the compiler and this is similar to
bpf_iter_css_{new,next,destroy}() functions where no other #ifdef have
been added in kernel/bpf/helpers.c and in the selftests.

Fixes: 9c66dc9 ("bpf: Introduce css_task open-coded iterator kfuncs")
Link: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/6665206927
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310260528.aHWgVFqq-lkp@intel.com/
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>

kernel/bpf/task_iter.c

@@ -892,6 +892,8 @@ __bpf_kfunc void bpf_iter_task_vma_destroy(struct bpf_iter_task_vma *it)
 
 __diag_pop();
 
+#ifdef CONFIG_CGROUPS
+
 struct bpf_iter_css_task {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));
@@ -950,6 +952,8 @@ __bpf_kfunc void bpf_iter_css_task_destroy(struct bpf_iter_css_task *it)
 
 __diag_pop();
 
+#endif /* CONFIG_CGROUPS */
+
 struct bpf_iter_task {
 	__u64 __opaque[3];
 } __attribute__((aligned(8)));