netfilter: nf_tables: reject constant set with timeout

This set combination is weird: it allows for elements to be
added/deleted, but once bound to the rule it cannot be updated anymore.
Eventually, all elements expire, leading to an empty set which cannot
be updated anymore. Reject this flags combination.

Cc: stable@vger.kernel.org
Fixes: 761da293

 ("netfilter: nf_tables: add set timeout API support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>