Original MSS clamp patch by Vladimir Ratnikov.
Modified to apply only to TCP packets.
Type: fix
Test: Wrote new unit test to ensure MAP-E TCP MSS clamping.
Change-Id: I77c69714a2e3cc3b7473aa12b645243bb595c2a2 Signed-off-by: Vladimir Ratnikov <vratnikov@netgate.com> Signed-off-by: Jon Loeliger <jdl@netgate.com>