[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
ac18e750 ([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>