selftests/bpf: Add test passing MAYBE_NULL reg to bpf_refcount_acquire

The test added in this patch exercises the logic fixed in the previous
patch in this series. Before the previous patch's changes,
bpf_refcount_acquire accepts MAYBE_NULL local kptrs; after the change
the verifier correctly rejects the such a call.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20231107085639.3016113-3-davemarchevsky@fb.com


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