netfilter: nf_tables: don't unregister hook when table is dormant

When nf_tables_updchain encounters an error, hook registration needs to
be rolled back.

This should only be done if the hook has been registered, which won't
happen when the table is flagged as dormant (inactive).

Just move the assignment into the registration block.

Reported-by:  <syzbot+53ed3a6440173ddbf499@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=53ed3a6440173ddbf499


Fixes: b9703ed4 ("netfilter: nf_tables: support for adding new devices to an existing netdev chain")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>