selftests/bpf: Test bpf_refcount_acquire of node obtained via direct ld

This patch demonstrates that verifier changes earlier in this series
result in bpf_refcount_acquire(mapval->stashed_kptr) passing
verification. The added test additionally validates that stashing a kptr
in mapval and - in a separate BPF program - refcount_acquiring the kptr
without unstashing works as expected at runtime.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20231107085639.3016113-7-davemarchevsky@fb.com


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