netfilter: netns ebtables: more cleanup during ebt_unregister_table()

Now that ebt_unregister_table() can be called during netns stop, and module
pinning scheme can't prevent netns stop, do table cleanup by hand.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>