ip: ip_options_compile() resilient to NULL skb route

Scot Doyle demonstrated ip_options_compile() could be called with an skb
without an attached route, using a setup involving a bridge, netfilter,
and forged IP packets.

Let's make ip_options_compile() and ip_options_rcv_srr() a bit more
robust, instead of changing bridge/netfilter code.

With help from Hiroaki SHIMODA.

Reported-by: Scot Doyle <[e-mail]>
Tested-by: Scot Doyle <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Acked-by: Hiroaki SHIMODA <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_options.c

···
 					pp_ptr = optptr + 2;
 					goto error;
 				}
-				if (skb) {
+				if (rt) {
 					memcpy(&optptr[optptr[2]-1], &rt->rt_spec_dst, 4);
 					opt->is_changed = 1;
 				}
···
 						goto error;
 					}
 					opt->ts = optptr - iph;
-					if (skb) {
+					if (rt)  {
 						memcpy(&optptr[optptr[2]-1], &rt->rt_spec_dst, 4);
 						timeptr = (__be32*)&optptr[optptr[2]+3];
 					}
···
 	unsigned long orefdst;
 	int err;
 
-	if (!opt->srr)
+	if (!opt->srr || !rt)
 		return 0;
 
 	if (skb->pkt_type != PACKET_HOST)