netfilter: nf_tables: free flowtable hooks on hook register error

If hook registration fails, the hooks allocated via nft_netdev_hook_alloc
need to be freed.

We can't change the goto label to 'goto 5' -- while it does fix the memleak
it does cause a warning splat from the netfilter core (the hooks were not
registered).

Fixes: 3f0465a9

 ("netfilter: nf_tables: dynamically allocate hooks per net_device in flowtables")
Reported-by:  <syzbot+a2ff6fa45162a5ed4dd3@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>