selftests/bpf: ringbuf_multi: Use runtime page size

Set bpf table sizes dynamically according to the runtime page size
value.

Do not switch to ASSERT macros, keep CHECK, for consistency with the
rest of the test. Can be a separate cleanup patch.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210408061310.95877-8-yauheni.kaliuta@redhat.com