net: tcp: Get rid of `recv_mss` field from TCP struct

This frees up some more memory as well, by computing the maximum segment
size whenever needed.  A flag is set in the TCP context to signal if
the value has been already computed.

Change-Id: Idb228d4682540f92b269e3878fcee45cbc28038a
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>