net: bridge: mcast: drop sizeof for nest attribute's zero size

This was a dumb error I made instead of writing nla_total_size(0)
for a nest attribute, I wrote nla_total_size(sizeof(0)).

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 606433fe3e11 ("net: bridge: mcast: dump ipv4 querier state")
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>