- don't try and add a PDU trailer (or TBD for that matter) to an M_EXT
mbuf. when M_EXT is set, the data area of the mbuf can be shared
with other mbufs (see m_copym()), and writing to one M_EXT mbuf may
invalidate another M_EXT mbuf (saw this with PDU trailers).
- be sure to validate the length value from the PDU trailer so that it
doesn't throw us out of sync with the chip (fixes yet another source
of the dread "en_service: id mismatch" panic).
- on recv, don't bother allocating an mbuf if mlen is zero since we are
only going to toss the data anyway.
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