[bpf2go] Generate union fields as fixed bytes array

[chenBenjamin97]

When bpf2go generates a union field into a Go field, it seeme like it takes the first field from the C union, then adds anonymous field with the rest of the union's size.

Example given:
The following C union:

  union {
    proc_info child; // child is used for process spawning events
    mmap_event mmap; // mmap is used for mmap events
  } raw_event;

is converted into the following Go field:

RawEvent struct {
    Child mainProcInfo
    _     [24]byte
}

This Go implementation might be problematic for union fields that are being used as a container for various possible structs.
For example, a generic struct that is being sent to the user-space through a ring buffer. Such a struct would probably hold an ID, indicating the user-space the context of the message, then the user-space code would parse the union field into the relevant Go struct (again, based on the context).

Current implementation requires two runtime operations in order to achieve this goal: first, convert the whole RawEvent field into a bytes array, then parse this array to the relevant Go struct.

If we'll just set the RawEvent Go field a fixed bytes array, we can avoid the first operation in runtime.

I guess we can add the suggested behavior as a flag to the bpf2go tool in order to maintain its backward compatibility.

What do you think about that?

If agreed, I'll be glad to implement it.

Thanks!