bpf, btf: Warn but return no error for NULL btf from __register_btf_kfunc_id_set()

__register_btf_kfunc_id_set() assumes .BTF to be part of the module's .ko
file if CONFIG_DEBUG_INFO_BTF is enabled. If that's not the case, the
function prints an error message and return an error. As a result, such
modules cannot be loaded.

However, the section could be stripped out during a build process. It would
be better to let the modules loaded, because their basic functionalities
have no problem [0], though the BTF functionalities will not be supported.
Make the function to lower the level of the message from error to warn, and
return no error.

  [0] https://lore.kernel.org/bpf/20220219082037.ow2kbq5brktf4f2u@apollo.legion

Fixes: c446fdacb10d ("bpf: fix register_btf_kfunc_id_set for !CONFIG_DEBUG_INFO_BTF")
Reported-by: Alexander Egorenkov <Alexander.Egorenkov@ibm.com>
Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/87y228q66f.fsf@oc8242746057.ibm.com
Link: https://lore.kernel.org/bpf/20220219082037.ow2kbq5brktf4f2u@apollo.legion
Link: https://lore.kernel.org/bpf/20230701171447.56464-1-sj@kernel.org

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