Enable prefetching of descriptors on the TX ring, using the same

values as in the Intel driver 3.8.21 for linux.  The fact that it
is standard in the above driver suggests that it has no bad side
effects.

But of course there must be a reason for enabling features, not
just "it does not harm", so here it is a good one:

Prefetching enables full line rate even using a single queue (14.88
Mpps, compared to ~12 Mpps without prefetch).  This in turn is
terribly useful when one wants to schedule traffic.

For obvious reasons the difference is only visible with netmap
or other high speed solutions, but presumably the advantage
should be in the order of a fraction of a microsecond when
starting transmission on an empty queue.

Discussed with Jack Vogel.

MFC after:	1 week

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