test_snprintf_btf fails on s390, because NULL points to a readable
struct lowcore there. Fix by using the last page instead.

Error message example:

    printing fffffffffffff000 should generate error, got (361)

Fixes: 076a95f5 ("selftests/bpf: Add bpf_snprintf_btf helper tests")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210227051726.121256-1-iii@linux.ibm.com