udp: assign flowid to udp sockets round-robin
On a 2x8x2 SKL this increases measured throughput with 64
netperf -H $DUT -t UDP_STREAM -- -m 1
from 590kpps to 1.1Mpps
before:
https://people.freebsd.org/~mmacy/2018.05.11/udpsender.svg
after:
https://people.freebsd.org/~mmacy/2018.05.11/udpsender2.svg