bpf: Add missing btf_put to register_btf_id_dtor_kfuncs

We take the BTF reference before we register dtors and we need
to put it back when it's done.

We probably won't se a problem with kernel BTF, but module BTF
would stay loaded (because of the extra ref) even when its module
is removed.

Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Fixes: 5ce937d6

 ("bpf: Populate pairs of btf_id and destructor kfunc in btf")
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230120122148.1522359-1-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>