net: bridge: fix memleak in br_add_if()

net: bridge: fix memleak in br_add_if()

I got a memleak report:

BUG: memory leak
unreferenced object 0x607ee521a658 (size 240):
comm "syz-executor.0", pid 955, jiffies 4294780569 (age 16.449s)
hex dump (first 32 bytes, cpu 1):
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace:
[<00000000d830ea5a>] br_multicast_add_port+0x1c2/0x300 net/bridge/br_multicast.c:1693
[<00000000274d9a71>] new_nbp net/bridge/br_if.c:435 [inline]
[<00000000274d9a71>] br_add_if+0x670/0x1740 net/bridge/br_if.c:611
[<0000000012ce888e>] do_set_master net/core/rtnetlink.c:2513 [inline]
[<0000000012ce888e>] do_set_master+0x1aa/0x210 net/core/rtnetlink.c:2487
[<0000000099d1cafc>] __rtnl_newlink+0x1095/0x13e0 net/core/rtnetlink.c:3457
[<00000000a01facc0>] rtnl_newlink+0x64/0xa0 net/core/rtnetlink.c:3488
[<00000000acc9186c>] rtnetlink_rcv_msg+0x369/0xa10 net/core/rtnetlink.c:5550
[<00000000d4aabb9c>] netlink_rcv_skb+0x134/0x3d0 net/netlink/af_netlink.c:2504
[<00000000bc2e12a3>] netlink_unicast_kernel net/netlink/af_netlink.c:1314 [inline]
[<00000000bc2e12a3>] netlink_unicast+0x4a0/0x6a0 net/netlink/af_netlink.c:1340
[<00000000e4dc2d0e>] netlink_sendmsg+0x789/0xc70 net/netlink/af_netlink.c:1929
[<000000000d22c8b3>] sock_sendmsg_nosec net/socket.c:654 [inline]
[<000000000d22c8b3>] sock_sendmsg+0x139/0x170 net/socket.c:674
[<00000000e281417a>] ____sys_sendmsg+0x658/0x7d0 net/socket.c:2350
[<00000000237aa2ab>] ___sys_sendmsg+0xf8/0x170 net/socket.c:2404
[<000000004f2dc381>] __sys_sendmsg+0xd3/0x190 net/socket.c:2433
[<0000000005feca6c>] do_syscall_64+0x37/0x90 arch/x86/entry/common.c:47
[<000000007304477d>] entry_SYSCALL_64_after_hwframe+0x44/0xae

On error path of br_add_if(), p->mcast_stats allocated in
new_nbp() need be freed, or it will be leaked.

Fixes: 6774caed7674 ("net: bridge: add support for IGMP/MLD stats and export them via netlink")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Link: https://lore.kernel.org/r/20210809132023.978546-1-yangyingliang@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>