Currently, to avoid generating too much BTF types, for a struct type
like
```
struct foo {
int val;
struct bar *ptr;
};
```
if the BTF generation reaches 'struct foo', it will not generate actual
type for 'struct bar' and instead a forward decl is generated. The
'struct bar' is actual generated in BTF unless it is reached through a
non-struct pointer member.
Such a limitation forces bpf developer to hack and workaround this
problem. See [1] and [2]. For example in [1], we have
```
struct map_value {
struct prog_test_ref_kfunc *not_kptr;
struct prog_test_ref_kfunc __kptr *val;
struct node_data __kptr *node;
};
```
The BTF type for 'struct node_data' is not generated. Note that we have
a '__kptr' annotation. Similar problem for [2] with a '__uptr'
annotation. Note that the issue in [1] has been resolved later but the
hack in [2] is still needed.
This patch relaxed the struct type (with struct pointer member) BTF
generation if the struct pointer has a btf_type_tag annotation.
[1] https://lore.kernel.org/r/20230310230743.2320707-4-davemarchevsky@fb.com
[2] https://lore.kernel.org/r/20241023234759.860539-9-martin.lau@linux.dev
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