Move some `struct nf_conntrack` code from linux/skbuff.h to
linux/nf_conntrack_common.h.  Together with a couple of helpers for
getting and setting skb->_nfct, it allows us to remove
CONFIG_NF_CONNTRACK checks from net/netfilter/nf_conntrack.h.
Signed-off-by: NJeremy Sowden <jeremy@azazel.net>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>