net: tcp: Handle storage of TCP send MSS

MSS is Maximum Segment Size (data payload) of TCP. In SYN packets,
each side of the connection shares an MSS it wants to use (receive)
via the corresponding TCP option. If the option is not available,
the RFC mandates use of the value 536.

This patch handles storage of the send MSS (in the TCP structure,
in TCP backlog), with follow up patch handling actual parsing it
from the SYN TCP options.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>