if the mbuf has a valid flowid, use it instead of using siphash24

if the mbuf has a valid flowid, use it instead of using siphash24
and a bunch of header fields we have to parse the mbuf for.

siphash24 is about 20% of the cost of sending a udp packet on a
trunk interface with tcpbench on my box. if there's a flowid set
we get all that back.

ok mpi@ mikeb@ sthen@