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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