netfilter: nft_compat: destroy function must not have side effects

netfilter: nft_compat: destroy function must not have side effects

The nft_compat destroy function deletes the nft_xt object from a list.
This isn't allowed anymore. Destroy functions are called asynchronously,
i.e. next batch can find the object that has a pending ->destroy()
invocation:

cpu0                       cpu1
 worker
   ->destroy               for_each_entry()
	                     if (x == ...
			        return x->ops;
     list_del(x)
     kfree_rcu(x)
                           expr->ops->... // ops was free'd

To resolve this, the list_del needs to occur before the transaction
mutex gets released.  nf_tables has a 'deactivate' hook for this
purpose, so use that to unlink the object from the list.

Fixes: 0935d5588400 ("netfilter: nf_tables: asynchronous release")
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>