netfilter: nf_tables: elements with timeout below CONFIG_HZ never expire

Element timeout that is below CONFIG_HZ never expires because the
timeout extension is not allocated given that nf_msecs_to_jiffies64()
returns 0. Set timeout to the minimum value to honor timeout.

Fixes: 8e1102d5 ("netfilter: nf_tables: support timeouts larger than 23 days")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>