[NETFILTER]: remove bogus memset() calls from ip_conntrack_netlink.c

nfattr_parse_nested() calls nfattr_parse() which in turn does a memset
on the 'tb' array.  All callers therefore don't need to memset before
calling it.

Signed-off-by: Pablo Neira <pablo@eurodev.net>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>