Explicitly allocate stack memory for ICMP payload in IPv4 forward.

Explicitly allocate stack memory for ICMP payload in IPv4 forward.

Old ip_forward() allocated a fake mbuf copy on the stack to send
an ICMP packet after ip_output() has failed.  It seems easier to
just copy the data onto the stack that icmp_error() may use.  Only
if the ICMP error packet is acutally sent, create the mbuf.

m_dup_pkthdr() uses atomic operation to link the incpb to mbuf.
pf_pkt_addr_changed() was immediately called afterwards to remove
the linkage again.  Also m_tag_delete_chain() was overhead.  New
code uses less CPU locking in the hot path.

OK deraadt@ claudio@