netns: oops in ip[6]_frag_reasm incrementing stats

dev can be NULL in ip[6]_frag_reasm for skb's coming from RAW sockets.

Quagga's OSPFD sends fragmented packets on a RAW socket, when netfilter
conntrack reassembles them on the OUTPUT path you hit this code path.

You can test it with something like "hping2 -0 -d 2000 -f AA.BB.CC.DD"

With help from Jarek Poplawski.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David S. Miller <davem@davemloft.net>