net: tcp: Precompute appdata properly

TCP packets need the computed packet length early so they can fill in
a correct sequence number in their generated ACKs.  Waiting for
packet_received() is too late.  Precompute it before needed, and skip
the evaluation later.

Change-Id: I25547009f88277e0042c74f2005a141819797886
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>