selftests/bpf: Test a uptr struct spanning across pages.

This patch tests the case when uptr has a struct spanning across two
pages. It is not supported now and EOPNOTSUPP is expected from the
syscall update_elem.

It also tests the whole uptr struct located exactly at the
end of a page and ensures that this case is accepted by update_elem.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20241023234759.860539-10-martin.lau@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>