selftests/bpf: Test overwriting referenced dynptr

selftests/bpf: Test overwriting referenced dynptr

Test overwriting referenced dynptr and clones to make sure it is only
allow when there is at least one other dynptr with the same ref_obj_id.
Also make sure slice is still invalidated after the dynptr's stack slot
is destroyed.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260406150548.1354271-3-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>