netfilter: ebtables: fix table blob use-after-free

We are not allowed to return an error at this point.
Looking at the code it looks like ret is always 0 at this
point, but its not.

t = find_table_lock(net, repl->name, &ret, &ebt_mutex);

... this can return a valid table, with ret != 0.

This bug causes update of table->private with the new
blob, but then frees the blob right away in the caller.

Syzbot report:

BUG: KASAN: vmalloc-out-of-bounds in __ebt_unregister_table+0xc00/0xcd0 net/bridge/netfilter/ebtables.c:1168
Read of size 4 at addr ffffc90005425000 by task kworker/u4:4/74
Workqueue: netns cleanup_net
Call Trace:
 kasan_report+0xbf/0x1f0 mm/kasan/report.c:517
 __ebt_unregister_table+0xc00/0xcd0 net/bridge/netfilter/ebtables.c:1168
 ebt_unregister_table+0x35/0x40 net/bridge/netfilter/ebtables.c:1372
 ops_exit_list+0xb0/0x170 net/core/net_namespace.c:169
 cleanup_net+0x4ee/0xb10 net/core/net_namespace.c:613
...

ip(6)tables appears to be ok (ret should be 0 at this point) but make
this more obvious.

Fixes: c58dd2dd ("netfilter: Can't fail and free after table replacement")
Reported-by:  <syzbot+f61594de72d6705aea03@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>