netfilter: ipset: remove useless memset() calls

One of the memset call is buggy: it does not erase full array, but only pointer size.
Moreover, after a check, first step of nla_parse_nested/nla_parse is to
erase tb array as well. We can remove both calls safely.

Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>