bpf: Remove useless else if

The assignment of the else and else if branches is the same, so the else
if here is redundant, so we remove it and add a comment to make the code
here readable.

./kernel/bpf/cgroup_iter.c:81:6-8: WARNING: possible condition with no effect (if == else).

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2016
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220831021618.86770-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/cgroup_iter.c

@@ -78,9 +78,7 @@ static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
 		return css_next_descendant_pre(NULL, p->start_css);
 	else if (p->order == BPF_CGROUP_ITER_DESCENDANTS_POST)
 		return css_next_descendant_post(NULL, p->start_css);
-	else if (p->order == BPF_CGROUP_ITER_ANCESTORS_UP)
-		return p->start_css;
-	else /* BPF_CGROUP_ITER_SELF_ONLY */
+	else /* BPF_CGROUP_ITER_SELF_ONLY and BPF_CGROUP_ITER_ANCESTORS_UP */
 		return p->start_css;
 }