netlink: fix state reallocation in policy export

Evidently, when I did this previously, we didn't have more than
10 policies and didn't run into the reallocation path, because
it's missing a memset() for the unused policies. Fix that.

Fixes: d07dcf9a ("netlink: add infrastructure to expose policies to userspace")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>