netfilter: nf_tables: fix set selection when timeouts are requested

The NFT_SET_TIMEOUT flag is ignore in nft_select_set_ops, which may
lead to selection of a set implementation that doesn't actually
support timeouts.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>