netfilter: nf_tables: reject element expiration with no timeout

If element timeout is unset and set provides no default timeout, the
element expiration is silently ignored, reject this instead to let user
know this is unsupported.

Also prepare for supporting timeout that never expire, where zero
timeout and expiration must be also rejected.

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