ip4: fix swapped src/dst params with ip4_route_src

This fixes a couple of occurrences where the src and dst parameters to
ip4_route_src() were swapped. This was most likely due to confusion between
ip_route(src, dst) and ip4_route_src(dst, src)

This was found in a system where LWIP_IPV4_SRC_ROUTING is 0

The UDP case was an application socket bound to INADDR_ANY with
IP_MULTICAST_IF set. Transmits would result in calling ip4_route(dst) where
dst was pcb->local_addr (which was INADDR_ANY) instead of pcb->mcast_ip4.
This resulted in a routing failure

The ICMP issue was found through code analysis only

src/core/ipv4/icmp.c

@@ -380,7 +380,7 @@ icmp_send_response(struct pbuf *p, u8_t type, u8_t code)
   {
     ip4_addr_t iphdr_dst;
     ip4_addr_copy(iphdr_dst, iphdr->dest);
-    netif = ip4_route_src(&iphdr_src, &iphdr_dst);
+    netif = ip4_route_src(&iphdr_dst, &iphdr_src);
   }
 #else
   netif = ip4_route(&iphdr_src);