libbpf: Fix func leak in attach_kprobe

Add missing free() for func pointer in attach_kprobe function.

Fixes: a2488b5f ("libbpf: Allow specification of "kprobe/function+offset"")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20210721215810.889975-2-jolsa@kernel.org

tools/lib/bpf/libbpf.c

@@ -10431,6 +10431,7 @@ static struct bpf_link *attach_kprobe(const struct bpf_sec_def *sec,
 		return libbpf_err_ptr(err);
 	}
 	if (opts.retprobe && offset != 0) {
+		free(func);
 		err = -EINVAL;
 		pr_warn("kretprobes do not support offset specification\n");
 		return libbpf_err_ptr(err);