net/packet: Fix a comment about network_header

skb->nh.raw has been renamed as skb->network_header in 2007, in
commit b0e380b1 ("[SK_BUFF]: unions of just one member don't get
                      anything done, kill them")

So here we change it to the new name.

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>