selftests/bpf: ringbuf: Use runtime page size

Replace hardcoded 4096 with runtime value in the userspace part of
the test and set bpf table sizes dynamically according to the 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/20210408061310.95877-6-yauheni.kaliuta@redhat.com