Consider the following situation: 1. A packet comes in that is to be forwarded 2. The destination of the packet is rewritten by some firewall code 3. The next link's MTU is too small 4. The packet has the DF bit set
Consider the following situation:
1. A packet comes in that is to be forwarded
2. The destination of the packet is rewritten by some firewall code
3. The next link's MTU is too small
4. The packet has the DF bit set

Then the current code is such that instead of setting the next
link's MTU in the ICMP error, ip_next_mtu() is called and a guess
is sent as to which MTU is supposed to be tried next. This is because
in this case ip_forward() is called with srcrt set to 1. In that
case the ia pointer remains NULL but it is needed to get the MTU
of the interface the packet is to be sent out from.
Thus, we always set ia to the outgoing interface.

MFC after:	2 weeks

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