net: tcp: Properly ACK received TCP data

Parse and validate the sequence number and send the ACK synchronously
in the tcp_established() callback.  No attempt is made to unify the
packet with any queued data (which isn't queued yet, but might be
someday).

Note that this requires that an accepted context clone over the
sequence numbers from the listening context (which received them
during the SYN).

(Note also that this exposes a bug in the current SYN handling:
there's only space for one set of sequence numbers on that listening
context, so if we get two SYNs before the round trip ACK arrives we'll
"accept" the wrong connection with garbage sequence numbers and both
connections will fail).

Change-Id: I129e2a4cd926385511a1d22eea6d4f19ff185501
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>