Heavily optimize the case of small RX packets of 160 bytes or less. For

this case, allocate a plain mbuf and copy the frame into it, then send the
copy up the stack, leaving the original mbuf+cluster in place in the
receive ring for immediate re-use.  This saves a trip through 2 of the
3 zones of the compound mbuf allocator, a trip through busdma, and a trip
through the 1 of the 3 mbuf destructors.  For our load at Netflix, this can
lower CPU consumption by as much as 20%.  The copy algorithm is based on
investigative work from Luigi Rizzo earlier in the year.

Reviewed by:	jfv
Obtained from:	Netflix

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