libbpf: Fix ring_buffer__poll() to return number of consumed samples

Fix ring_buffer__poll() to return the number of non-discarded records
consumed, just like its documentation states. It's also consistent with
ring_buffer__consume() return. Fix up selftests with wrong expected results.

Fixes: bf99c936f9 ("libbpf: Add BPF ring buffer support")
Fixes: cb1c9ddd55 ("selftests/bpf: Add BPF ringbuf selftests")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201130223336.904192-1-andrii@kernel.org

tools/lib/bpf/ringbuf.c

@@ -278,7 +278,7 @@ int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 		err = ringbuf_process_ring(ring);
 		if (err < 0)
 			return err;
-		res += cnt;
+		res += err;
 	}
 	return cnt < 0 ? -errno : res;
 }