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