resolve_btfids: Support for KF_IMPLICIT_ARGS

resolve_btfids: Support for KF_IMPLICIT_ARGS

Implement BTF modifications in resolve_btfids to support BPF kernel
functions with implicit arguments.

For a kfunc marked with KF_IMPLICIT_ARGS flag, a new function
prototype is added to BTF that does not have implicit arguments. The
kfunc's prototype is then updated to a new one in BTF. This prototype
is the intended interface for the BPF programs.

A <func_name>_impl function is added to BTF to make the original kfunc
prototype searchable for the BPF verifier. If a <func_name>_impl
function already exists in BTF, its interpreted as a legacy case, and
this step is skipped.

Whether an argument is implicit is determined by its type:
currently only `struct bpf_prog_aux *` is supported.

As a result, the BTF associated with kfunc is changed from

    __bpf_kfunc bpf_foo(int arg1, struct bpf_prog_aux *aux);

into

    bpf_foo_impl(int arg1, struct bpf_prog_aux *aux);
    __bpf_kfunc bpf_foo(int arg1);

For more context see previous discussions and patches [1][2].

[1] https://lore.kernel.org/dwarves/ba1650aa-fafd-49a8-bea4-bdddee7c38c9@linux.dev/
[2] https://lore.kernel.org/bpf/20251029190113.3323406-1-ihor.solodrai@linux.dev/

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260120222638.3976562-6-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>