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