The protocol is:

  request: read (with buffer 1024)
  response: data (if buffer is big enough), or (data size, ERANGE)

Before, the raw API provided

* GetXAttrSize: called if the kernel sends a 0 sized buffer (which
  never happens

* GetXAttrData: called in other cases. The return value is []byte
  forcing allocation of potentially large buffers

* ListXAttr: return value []byte, so also problematic for allocation.

Now, the raw API mirrors the kernel API more closely, and use recycled
buffers for more efficiency.