When pf modifies a TCP packet, it sets the M_TCP_CSUM_OUT flag in

the mbuf packet header.  If the packet and is later dropped in
ip6_forward(), the TCP mbuf is copied and passed to icmp6_error().
IPv6 uses m_copym() and M_PREPEND() which preserve the packet header.
The inherited M_TCP_CSUM_OUT flag generates ICMP6 packets with an
incorrect checksum.  So reset the csum_flags when packets are
generated by icmp6_reflect() or icmp6_redirect_output().

IPv4 does m_copydata() into a fresh mbuf.  There m_inithdr() clears
the packet header, so the problem does not occur.  But setting the
csum_flags explicitly also makes sense for icmp_send().  Do not or
M_ICMP_CSUM_OUT to a value that is 0 because of some function calls
before.

OK mpi@ lteo@

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