bpf: Fix uninitialized symbol in bpf_perf_link_fill_kprobe()

The patch 1b715e1: "bpf: Support ->fill_link_info for
perf_event" from Jul 9, 2023, leads to the following Smatch static
checker warning:

    kernel/bpf/syscall.c:3416 bpf_perf_link_fill_kprobe()
    error: uninitialized symbol 'type'.

That can happens when uname is NULL. So fix it by verifying the uname
when we really need to fill it.

Fixes: 1b715e1 ("bpf: Support ->fill_link_info for perf_event")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/85697a7e-f897-4f74-8b43-82721bebc462@kili.mountain/
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

kernel/bpf/syscall.c

@@ -3378,14 +3378,14 @@ static int bpf_perf_link_fill_common(const struct perf_event *event,
 
 	if (!ulen ^ !uname)
 		return -EINVAL;
-	if (!uname)
-		return 0;
 
 	err = bpf_get_perf_event_info(event, &prog_id, fd_type, &buf,
 				      probe_offset, probe_addr);
 	if (err)
 		return err;
 
+	if (!uname)
+		return 0;
 	if (buf) {
 		len = strlen(buf);
 		err = bpf_copy_to_user(uname, buf, ulen, len);