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

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: 56ad61d8aa95 ("libbpf: Add BPF ring buffer support")
Fixes: b7d03ba86d3a ("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