bpf: Reject struct_ops registration that uses module ptr and the module btf_id is missing

In the Linux kernel, the following vulnerability has been resolved:

bpf: Reject struct_ops registration that uses module ptr and the module btf_id is missing

There is a UAF report in the bpf_struct_ops when CONFIG_MODULES=n.
In particular, the report is on tcp_congestion_ops that has
a "struct module *owner" member.

For struct_ops that has a "struct module *owner" member,
it can be extended either by the regular kernel module or
by the bpf_struct_ops. bpf_try_module_get() will be used
to do the refcounting and different refcount is done
based on the owner pointer. When CONFIG_MODULES=n,
the btf_id of the "struct module" is missing:

WARN: resolve_btfids: unresolved symbol module

Thus, the bpf_try_module_get() cannot do the correct refcounting.

Not all subsystem's struct_ops requires the "struct module *owner" member.
e.g. the recent sched_ext_ops.

This patch is to disable bpf_struct_ops registration if
the struct_ops has the "struct module *" member and the
"struct module" btf_id is missing. The btf_type_is_fwd() helper
is moved to the btf.h header file for this test.

This has happened since the beginning of bpf_struct_ops which has gone
through many changes. The Fixes tag is set to a recent commit that this
patch can apply cleanly. Considering CONFIG_MODULES=n is not
common and the age of the issue, targeting for bpf-next also.