[IPV4]: Make ip_defrag return the same packet

This patch is a bit of a hack.  However it is worth it if you consider that
this is the only reason why we have to carry around the struct sk_buff **
pointers in netfilter.

It makes ip_defrag always return the packet that was given to it on input.
It does this by cloning the packet and replacing its original contents with
the head fragment if necessary.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>