[NET]: Reduce size of struct sk_buff on 64 bit architectures

Move skb->nf_mark next to skb->tc_index to remove a 4 byte hole between
skb->nfmark and skb->nfct and another one between skb->users and skb->head
when CONFIG_NETFILTER, CONFIG_NET_SCHED and CONFIG_NET_CLS_ACT are enabled.
For all other combinations the size stays the same.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>