When I created UDP socket splicing, I added the goto nextpkt loop

When I created UDP socket splicing, I added the goto nextpkt loop
to splice multiple UDP packets in the m_nextpkt list.  Some profiling
with TCP splicing showed that checking so_rcv.sb_mb is wrong.  It
causes several useless runs through the loop.  Better check for
nextrecord which contains the original m_nextpkt value of the mbuf.
OK mikeb@