ipv4: fix path MTU discovery with connection tracking

IPv4 conntrack defragments incoming packet at the PRE_ROUTING hook and
(in case of forwarded packets) refragments them at POST_ROUTING
independent of the IP_DF flag. Refragmentation uses the dst_mtu() of
the local route without caring about the original fragment sizes,
thereby breaking PMTUD.

This patch fixes this by keeping track of the largest received fragment
with IP_DF set and generates an ICMP fragmentation required error during
refragmentation if that size exceeds the MTU.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: David S. Miller <davem@davemloft.net>

include/net/inet_frag.h

@@ -29,6 +29,8 @@ struct inet_frag_queue {
 #define INET_FRAG_COMPLETE	4
 #define INET_FRAG_FIRST_IN	2
 #define INET_FRAG_LAST_IN	1
+
+	u16			max_size;
 };
 
 #define INETFRAGS_HASHSZ		64

include/net/ip.h

@@ -42,6 +42,8 @@ struct inet_skb_parm {
 #define IPSKB_XFRM_TRANSFORMED	4
 #define IPSKB_FRAG_COMPLETE	8
 #define IPSKB_REROUTED		16
+
+	u16			frag_max_size;
 };
 
 static inline unsigned int ip_hdrlen(const struct sk_buff *skb)