net: tcp: Correctly handle partial duplicate out of order data

TCP is a streaming protocol, this means it can set the packet boundary
at an arbitrary location. Therefor a re-transmitted packet may have the
packet boundary at a different location from the original missing segment.

The reordering logic should be able to deal with this situation and
throw away the overlapping data.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>