net: tcp: Refactor "finalize" step from "prepare"

The ACK field of a packet depends on the received sequence number from
the other side of a connection, which can change long after a packet
is prepared/queued.  The IP finalize step (which computes things like
the TCP checksum) needs to be invoked just before a packet is sent,
not from prepare.

Split it out to a separate function.

Change-Id: Ibdae0c9b6833bdfb130cc381f823150dceab6637
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>