[NETFILTER]: Fix kernel panic with REDIRECT target.

When connection tracking entry (nf_conn) is about to copy itself it can
have some of its extension users (like nat) as being already freed and
thus not required to be copied.

Actually looking at this function I suspect it was copied from
nf_nat_setup_info() and thus bug was introduced.

Report and testing from David <david@unsolicited.net>.

[ Patrick McHardy states:

	I now understand whats happening:

	- new connection is allocated without helper
	- connection is REDIRECTed to localhost
	- nf_nat_setup_info adds NAT extension, but doesn't initialize it yet
	- nf_conntrack_alter_reply performs a helper lookup based on the
	   new tuple, finds the SIP helper and allocates a helper extension,
	   causing reallocation because of too little space
	- nf_nat_move_storage is called with the uninitialized nat extension

	So your fix is entirely correct, thanks a lot :)  ]
Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/nf_nat_core.c

@@ -607,13 +607,10 @@ static void nf_nat_move_storage(struct nf_conn *conntrack, void *old)
 	struct nf_conn_nat *new_nat = nf_ct_ext_find(conntrack, NF_CT_EXT_NAT);
 	struct nf_conn_nat *old_nat = (struct nf_conn_nat *)old;
 	struct nf_conn *ct = old_nat->ct;
-	unsigned int srchash;
 
-	if (!(ct->status & IPS_NAT_DONE_MASK))
+	if (!ct || !(ct->status & IPS_NAT_DONE_MASK))
 		return;
 
-	srchash = hash_by_src(&ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple);
-
 	write_lock_bh(&nf_nat_lock);
 	hlist_replace_rcu(&old_nat->bysource, &new_nat->bysource);
 	new_nat->ct = ct;