selftests/bpf: Fix return value checks in perf_event_stackmap test

The bpf_get_stackid() function may also return 0 on success as per UAPI BPF
helper documentation. Therefore, correct checks from 'val > 0' to 'val >= 0'
to ensure that they cover all possible success return values.
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220408041452.933944-1-ytcoode@gmail.com

tools/testing/selftests/bpf/progs/perf_event_stackmap.c

@@ -35,10 +35,10 @@ int oncpu(void *ctx)
 	long val;
 
 	val = bpf_get_stackid(ctx, &stackmap, 0);
-	if (val > 0)
+	if (val >= 0)
 		stackid_kernel = 2;
 	val = bpf_get_stackid(ctx, &stackmap, BPF_F_USER_STACK);
-	if (val > 0)
+	if (val >= 0)
 		stackid_user = 2;
 
 	trace = bpf_map_lookup_elem(&stackdata_map, &key);