[IPV4]: Fix null dereference in ip_defrag

Been seeing occasional panics in my testing of 2.6.25-rc in ip_defrag.
Offending line in ip_defrag is here:

	net = skb->dev->nd_net

where dev is NULL.  Bisected the problem down to commit
ac18e7509e7df327e30d6e073a787d922eaf211d ([NETNS][FRAGS]: Make the
inet_frag_queue lookup work in namespaces).  

Below patch (idea from Patrick McHardy) fixes the problem for me.

Signed-off-by: Phil Oester <kernel@linuxace.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_fragment.c

@@ -568,7 +568,7 @@ int ip_defrag(struct sk_buff *skb, u32 user)
 
 	IP_INC_STATS_BH(IPSTATS_MIB_REASMREQDS);
 
-	net = skb->dev->nd_net;
+	net = skb->dev ? skb->dev->nd_net : skb->dst->dev->nd_net;
 	/* Start by cleaning up the memory. */
 	if (atomic_read(&net->ipv4.frags.mem) > net->ipv4.frags.high_thresh)
 		ip_evictor(net);