[BPF] Allow struct pointee member btf generation with annotations (#141719)

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