[EBTABLES]: Verify that ebt_entries have zero ->distinguisher.

We need that for iterator to work; existing check had been too weak.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>