bpf: Unify referenced object tracking in verifier

Helpers and kfuncs independently tracked referenced object metadata
using standalone id fields in their respective arg_meta structs.
This led to duplicated logic and inconsistent error handling between the
two paths.

Introduce struct ref_obj_desc to consolidate id and parent_id along with
a count of how many arguments carry a reference. Add update_ref_obj() to
populate it from a bpf_reg_state, replacing open-coded assignments in
check_func_arg(), check_kfunc_args(), and process_iter_arg(). Add
validate_ref_obj() to check for ambiguous ref_obj before using it.

For ref_obj releasing helpers and kfuncs, keep checking it before
calling update_ref_obj() for now. A later patch will make these
functions not depending on ref_obj. For other users of ref_obj, move the
checks to the use locations. For helper, this means moving the checks
inside helper_multiple_ref_obj_use() to use locations.
is_acquire_function() is dropped as ref_obj is never used.

Pass ref_obj_desc into process_dynptr_func()/mark_stack_slots_dynptr()
instead of a bare parent_id to make it less confusing.

Drop the selftest introduced in 7ec899ac90a2 ("selftests/bpf: Negative
test case for ref_obj_id in args") since the verifier no longer
complains about ambiguous ref_obj if it is not used.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260529014936.2811085-8-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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