ip: discard IPv4 datagrams with overlapping segments.

ip: discard IPv4 datagrams with overlapping segments.

This behavior is required in IPv6, and there is little need
to tolerate overlapping fragments in IPv4. This change
simplifies the code and eliminates potential DDoS attack vectors.

Tested: ran ip_defrag selftest (not yet available uptream).

Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 7969e5c40dfd04799d4341f1b7cd266b6e47f227)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>