netfilter: nf_tables: set dormant flag on hook register failure

We need to set the dormant flag again if we fail to register
the hooks.

During memory pressure hook registration can fail and we end up
with a table marked as active but no registered hooks.

On table/base chain deletion, nf_tables will attempt to unregister
the hook again which yields a warn splat from the nftables core.

Reported-and-tested-by:  <syzbot+de4025c006ec68ac56fc@syzkaller.appspotmail.com>
Fixes: 179d9ba5 ("netfilter: nf_tables: fix table flag updates")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>