selftests/bpf: Add test cases for bpf_list_del/add/is_first/is_last/empty

selftests/bpf: Add test cases for bpf_list_del/add/is_first/is_last/empty

Extend refcounted_kptr with tests for bpf_list_add (including prev from
bpf_list_front and bpf_refcount_acquire), bpf_list_del (including node
from bpf_list_front, bpf_rbtree_remove and bpf_refcount_acquire),
bpf_list_empty, bpf_list_is_first/last, and push_back on uninit head.

To verify the validity of bpf_list_del/add, the test also expects the
verifier to reject calls to bpf_list_del/add made without holding the
spin_lock.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Link: https://lore.kernel.org/r/20260521032306.97118-9-kaitao.cheng@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>