It was possible for ip_forward() to supply to icmp_error()

an IP header with ip_len in network byte order. For certain
values of ip_len, this could cause icmp_error() to write
beyond the end of an mbuf, causing mbuf free-list corruption.
This problem was observed during generation of ICMP redirects.

We now make quite sure that the copy of the IP header kept
for icmp_error() is stored in a non-shared mbuf header so
that it will not be modified by ip_output().

Also:
- Calculate the correct number of bytes that need to be
  retained for icmp_error(), instead of assuming that 64
  is enough (it's not).
- In icmp_error(), use m_copydata instead of bcopy() to
  copy from the supplied mbuf chain, in case the first 8
  bytes of IP payload are not stored directly after the IP
  header.
- Sanity-check ip_len in icmp_error(), and panic if it is
  less than sizeof(struct ip). Incoming packets with bad
  ip_len values are discarded in ip_input(), so this should
  only be triggered by bugs in the code, not by bad packets.

This patch results from code and suggestions from Ruslan, Bosko,
Jonathan Lemon and Matt Dillon, with important testing by Mike
Tancsa, who could reproduce this problem at will.

Reported by:	Mike Tancsa <mike@sentex.net>
Reviewed by:	ru, bmilekic, jlemon, dillon

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