selftests/bpf: Add test case for rbtree nodes that contain both bpf_refcount and...

selftests/bpf: Add test case for rbtree nodes that contain both bpf_refcount and kptr fields.

Allow bpf_kptr_xchg to directly operate on pointers marked with
NON_OWN_REF | MEM_RCU.

In the example demonstrated in this patch, as long as "struct
bpf_refcount ref" exists, the __kptr pointer is guaranteed to
carry the MEM_RCU flag. The ref member itself does not need to
be explicitly used.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Link: https://lore.kernel.org/r/20260214124042.62229-6-pilgrimtao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>