skbuff: Move new code into __copy_skb_header

Hi:

skbuff: Move new __skb_clone code into __copy_skb_header

It seems that people just keep on adding stuff to __skb_clone
instead __copy_skb_header.  This is wrong as it means your brand-new
attributes won't always get copied as you intended.

This patch moves them to the right place, and adds a comment to
prevent this from happening again.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks,
Signed-off-by: David S. Miller <davem@davemloft.net>