selftests/bpf: Verifier test on refill from a smaller spill

This patch adds a verifier test to ensure the verifier can read 8 bytes
from the stack after two 32bit write at fp-4 and fp-8. The test is similar
to the reported case from bcc [0].

  [0] https://github.com/iovisor/bcc/pull/3683



Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211102064541.316414-1-kafai@fb.com