MFC r260871:

  If the flowid is available for the mbuf that finalised the creation
  of a syncache connection, copy it into the inp_flowid field.

  Without this, an incoming TCP connection won't have an inp_flowid marked
  until some data comes in, and this means that things like the per-CPU
  TCP timer option will choose a different CPU for the timer work.
  (It also means that if one grabbed the flowid via an ioctl from userland,
  it won't be available until some data has been received.)

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