netfilter: flowtable: avoid possible false sharing

The flowtable follows the same timeout approach as conntrack, use the
same idiom as in cc169213 ("netfilter: conntrack: avoid same-timeout
update") but also include the fix provided by e37542ba ("netfilter:
conntrack: avoid possible false sharing").

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>