Tiny step to keep ether header in mbuf on ethernet input path:

Tiny step to keep ether header in mbuf on ethernet input path:
- ng_ether_input_p does not need interface to release its serializer.
- Instead of letting ng_ether_input_p glue back ether header, we simply
  pass mbuf with ether header to it.  Change ng_ether_input_p interface
  accordingly.