IPv6 forward copies small packet content on the stack.

IPv6 forward copies small packet content on the stack.

Unfortunately RFC 4443 demands that the ICMP6 error packet containing
the orignal packet is up to 1280 bytes long.  That means for every
forwarded packet forward() creates a mbuf copy, just in case delivery
fails.

For small packets we can copy the content on the stack like IPv4
forward does.  This saves us some mbuf allocations if the content
is shorter than the mbuf data size.

OK mvs@