selftests/bpf: Fix ringbuf selftest sample counting undeterminism

Fix test race, in which background poll can get either 5 or 6 samples,
depending on timing of notification. Prevent this by open-coding sample
triggering and forcing notification for the very last sample only.

Also switch to using atomic increments and exchanges for more obviously
reliable counting and checking. Additionally, check expected processed sample
counters for single-threaded use cases as well.

Fixes: 9a5f25ad ("selftests/bpf: Fix sample_cnt shared between two threads")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200608003615.3549991-1-andriin@fb.com