netfilter: flowtable: reorder nf_flowtable struct members

Place the read-mostly parts accessed by the datapath first.

In particular, we do access ->flags member (to see if HW offload
is enabled) for every single packet, but this is placed in the 5th
cacheline.

priority could stay where it is, but move it too to cover a hole.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>