tcp: Enable symmetric hashing by setting hash on outgoing conns

Now that we can trust NICs to supply an identical hash result
to software, we can setup the inpcb hash on outgoing connections.
This gives us symmetric hashing, meaning packets should enter
and leave on the same NIC queue.

Differential Revision: https://reviews.freebsd.org/D53104
Reviewed by: adrian, cc, kbowling, tuexen, zlei
Sponsored by: Netflix