IP: Send an ICMP "Fragment Reassembly Timeout" message when enabling connection track

IP: Send an ICMP "Fragment Reassembly Timeout" message when enabling connection track

No matter whether connection track is enabled, an end host should send
an ICMPv4 "Fragment Reassembly Timeout" message when defrag timeout.
The reasons are following two points:

1. RFC 792 says:
   >>>> >> > >   If a host reassembling a fragmented datagram cannot complete the
   >>>> >> > >   reassembly due to missing fragments within its time limit it
   >>>> >> > >   discards the datagram, and it may send a time exceeded message.
   >>>> >> > >
   >>>> >> > >   If fragment zero is not available then no time exceeded need be
   >>>> >> > >   sent at all.
   >>>> >> > >
   >>>> >> > > Read more: http://www.faqs.org/rfcs/rfc792.html#ixzz0aOXRD7Wp

2. Patrick McHardy also agrees with this opinion.   :-)
   About the discussion of this opinion, refer to http://patchwork.ozlabs.org/patch/41649

The patch fixed the problem like this:
When enabling connection track, fragments are received at PRE_ROUTING HOOK.
If they are failed to reassemble, ip_expire() will be called.
Before sending an ICMP "Fragment Reassembly Timeout" message,
the patch searches router table to get the destination entry only for host type.

The patch has been tested on both host type and route type.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>