Under RSS, assign a TCP flow's inp_flowid anyway.

Summary:
This brings some benefit of a tcp flow identification for some kernel
modules, such as siftr.

Reviewers: rrs, rscheff, tuexen, transport!
Approved by: tuexen (mentor), rrs
Subscribers: imp, melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D40061