ipv4: more compliant RFC 3168 support

Commit 6623e3b2 (ipv4: IP defragmentation must be ECN aware) was an
attempt to not lose "Congestion Experienced" (CE) indications when
performing datagram defragmentation.

Stefanos Harhalakis raised the point that RFC 3168 requirements were not
completely met by this commit.

In particular, we MUST detect invalid combinations and eventually drop
illegal frames.

Reported-by: Stefanos Harhalakis <v13@v13.gr>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>