netfilter: nftables: fix netlink report logic in flowtable and genid

The netlink report should be sent regardless the available listeners.

Fixes: 84d7fce6 ("netfilter: nf_tables: export rule-set generation ID")
Fixes: 3b49e2e9

 ("netfilter: nf_tables: add flow table netlink frontend")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>