move the allocation of the mbuf copy to use for sending icmp errors

above the reroute label that pf uses after it does dest addr rewriting
on outgoing packets. previously rerouting caused an extra copy to
be allocated, which caused the first copy to be leaked.

this has been bugging me for over half a year probably. after i got
some rudimental mbuf tracing in place it took me and alex wilson
most of a work day to audit the stack.

ok claudio@ on an earlier diff
ok mpi@, who made my first diff better and was gracious enough to
let me commit his because of the pain id been through finding this
leak.
ok pelikan@

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