bpf: Resolve BTF ID of ARG_PTR_TO_BTF_ID in kfunc bpf_func_proto

check_kfunc_args() walks the kfunc's BTF on every verification of a call
to work out which BTF ID an ARG_PTR_TO_BTF_ID argument expects. Helpers
name theirs in bpf_func_proto::arg_btf_id[], as a pointer to a BTF ID
that resolve_btfids fills in at build time.

The ID of a kfunc argument's referent is already stored in the
immutable BTF records that describe its pointer and modifier chain. Make
arg_btf_id[] point to the BTF field containing the resolved ID. A __map
argument instead uses the existing vmlinux BTF ID pointer. Produce this
metadata alongside the argument classification in get_kfunc_arg_type().

Declare the argument BTF ID pointers const, since the verifier only
reads through them. The BTF object owns their storage and remains
alive while the generated prototype is used, so the pointers remain
valid when the kfunc descriptor array is reallocated or sorted.

Both helper and kfunc callers can now read the expected BTF ID through
the same bpf_func_proto field, which lets check_func_arg() take over
the ARG_PTR_TO_BTF_ID case.

arg_btf_id[] shares a union with arg_size[], so an argument cannot
store both. The scalar-struct memory fallback keeps resolving its byte
size at verification time, as it does today.

No functional change.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260911220415.1396439-13-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>