bpf: Don't use idx variable when registering kfunc dtors

In commit fda01efc ("bpf: Enable cgroups to be used as kptrs"), I
added an 'int idx' variable to kfunc_init() which was meant to
dynamically set the index of the btf id entries of the
'generic_dtor_ids' array. This was done to make the code slightly less
brittle as the struct cgroup * kptr kfuncs such as bpf_cgroup_aquire()
are compiled out if CONFIG_CGROUPS is not defined. This, however, causes
an lkp build warning:

>> kernel/bpf/helpers.c:2005:40: warning: multiple unsequenced
   modifications to 'idx' [-Wunsequenced]
	.btf_id       = generic_dtor_ids[idx++],

Fix the warning by just hard-coding the indices.

Fixes: fda01efc ("bpf: Enable cgroups to be used as kptrs")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Vernet <void@manifault.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221123135253.637525-1-void@manifault.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -2019,16 +2019,16 @@ static const struct btf_kfunc_id_set common_kfunc_set = {
 
 static int __init kfunc_init(void)
 {
-	int ret, idx = 0;
+	int ret;
 	const struct btf_id_dtor_kfunc generic_dtors[] = {
 		{
-			.btf_id       = generic_dtor_ids[idx++],
-			.kfunc_btf_id = generic_dtor_ids[idx++]
+			.btf_id       = generic_dtor_ids[0],
+			.kfunc_btf_id = generic_dtor_ids[1]
 		},
 #ifdef CONFIG_CGROUPS
 		{
-			.btf_id       = generic_dtor_ids[idx++],
-			.kfunc_btf_id = generic_dtor_ids[idx++]
+			.btf_id       = generic_dtor_ids[2],
+			.kfunc_btf_id = generic_dtor_ids[3]
 		},
 #endif
 	};