libbpf: Use probe_name for legacy kprobe

Fix a bug in commit 46ed5fc3, which wrongly used the
func_name instead of probe_name to register legacy kprobe.

Fixes: 46ed5fc3 ("libbpf: Refactor and simplify legacy kprobe code")
Co-developed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Qiang Wang <wangqiang.wq.frank@bytedance.com>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211227130713.66933-1-wangqiang.wq.frank@bytedance.com

tools/lib/bpf/libbpf.c

@@ -10017,7 +10017,7 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 		gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name),
 					     func_name, offset);
 
-		legacy_probe = strdup(func_name);
+		legacy_probe = strdup(probe_name);
 		if (!legacy_probe)
 			return libbpf_err_ptr(-ENOMEM);