netfilter: ipset: Fix range bug in hash:ip,port,net

Due to the missing ininitalization at adding/deleting entries, when
a plain_ip,port,net element was the object, multiple elements were
added/deleted instead. The bug came from the missing dangling
default initialization.

The error-prone default initialization is corrected in all hash:* types.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>