selftests/bpf: Test libbpf's ringbuf size fix up logic

Make sure we always excercise libbpf's ringbuf map size adjustment logic
by specifying non-zero size that's definitely not a page size multiple.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-10-andrii@kernel.org