bpf: Generate kfunc argument prototype at add-call time

Kfunc argument checking re-derives each argument's kfunc_ptr_arg_type
from BTF on every verification of a call in check_kfunc_args(). Now that
get_kfunc_arg_type() is a function of the kfunc's BTF alone, it no
longer inspects register state. The classification can be computed once
when the call is added and cached. This is a step toward describing
kfuncs with a bpf_func_proto and sharing the helper argument-checking
path.

Generate the classification at bpf_add_kfunc_call() time:

- Extend struct bpf_func_proto to be able to describe a kfunc: widen
  arg_type[] and the arg_btf_id[]/arg_size[] union from 5 to
  MAX_BPF_FUNC_ARGS, since a kfunc may take up to 12 arguments (5 in
  registers, 7 on the stack).

- Embed a bpf_func_proto in struct bpf_kfunc_desc, populated by
  gen_kfunc_arg_proto() which runs get_kfunc_arg_type() for each
  argument and stores the result in proto.arg_type[]. Grow the
  descriptor table's descs[] as a flexible array to not waste memory.

- check_kfunc_args() reads the cached classification from meta->fn

The KF_ARG_PTR_TO_CTX classification depends on the resolved program type,
and for BPF_PROG_TYPE_EXT that is the target program's type, which
resolve_prog_type() reads from prog->aux->saved_dst_prog_type. That field
is normally recorded later during verification in check_attach_btf_id(),
after bpf_add_kfunc_call() has run.

Record saved_dst_prog_type and saved_dst_attach_type from dst_prog at
program load time in bpf_prog_load() so the resolved type is available
at add-call time without reordering check_attach_btf_id(). This keeps
e.g. an freplace of an XDP program calling bpf_xdp_metadata_rx_hash()
classifying its struct xdp_md * argument as context.

The classification result is unchanged; it is only computed earlier and
cached.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/bpf/20260801074633.1595644-19-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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