bpf: Restore sysctl new-value from 1 to 0

Commit 4e63acdff8 ("bpf: Introduce bpf_sysctl_{get,set}_new_value
helpers") changed the success return value to 0, but failed to update the
corresponding check in __cgroup_bpf_run_filter_sysctl(). Since
bpf_prog_run_array_cg() now returns 0 on success, the legacy ret == 1
condition is never satisfied. As a result, the modified value is ignored,
and bpf_sysctl_set_new_value() fails to replace the write buffer.

Fix this by checking for a return value of 0 instead, so cgroup/sysctl
programs can correctly replace the pending sysctl buffer.

This bug was discovered during a manual code review. Tested via a
cgroup/sysctl BPF reproducer overriding writes to a target sysctl.
Pre-fix, bpf_sysctl_set_new_value("foo") was silently ignored: the write
returned 8192 and the value remained "600". Post-fix, the BPF replacement
buffer properly propagates: the write returns 3 and the value updates to
"foo".

Fixes: f10d059661 ("bpf: Make BPF_PROG_RUN_ARRAY return -err instead of allow boolean")
Cc: stable@vger.kernel.org

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Signed-off-by: Dawei Feng <dawei.feng@seu.edu.cn>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/20260603105317.944304-4-dawei.feng@seu.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/cgroup.c

@@ -1936,7 +1936,7 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
 
 	kfree(ctx.cur_val);
 
-	if (ret == 1 && ctx.new_updated) {
+	if (!ret && ctx.new_updated) {
 		kvfree(*buf);
 		*buf = ctx.new_val;
 		*pcount = ctx.new_len;