bpf: Remove a WARN_ON_ONCE warning related to local kptr

Currently, in function bpf_obj_free_fields(), for local kptr,
a warning will be issued if the struct does not contain any
special fields. But actually the kernel seems totally okay
with a local kptr without any special fields. Permitting
no special fields also aligns with future percpu kptr which
also allows no special fields.
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230824063417.201925-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/syscall.c

@@ -657,7 +657,6 @@ void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
 			if (!btf_is_kernel(field->kptr.btf)) {
 				pointee_struct_meta = btf_find_struct_meta(field->kptr.btf,
 									   field->kptr.btf_id);
-				WARN_ON_ONCE(!pointee_struct_meta);
 				migrate_disable();
 				__bpf_obj_drop_impl(xchgd_field, pointee_struct_meta ?
 								 pointee_struct_meta->record :