net: bridge: fix a possible memory leak in __vlan_add

After per-port vlan stats, vlan stats should be released
when fail to add vlan

Fixes: 9163a0fc

 ("net: bridge: add support for per-port vlan stats")
CC: bridge@lists.linux-foundation.org
cc: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
CC: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>