bridge: Fix memory leak when deleting bridge with vlan filtering enabled

We currently don't call br_vlan_flush() when deleting a bridge, which
leads to memory leak if br->vlan_info is allocated.

Steps to reproduce:
  while :
  do
    brctl addbr br0
    bridge vlan add dev br0 vid 10 self
    brctl delbr br0
  done
We can observe the cache size of corresponding slab entry
(as kmalloc-2048 in SLUB) is increased.

kmemleak output:
unreferenced object 0xffff8800b68a7000 (size 2048):
  comm "bridge", pid 2086, jiffies 4295774704 (age 47.656s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 48 9b 36 00 88 ff ff  .........H.6....
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff815eb6ae>] kmemleak_alloc+0x4e/0xb0
    [<ffffffff8116a1ca>] kmem_cache_alloc_trace+0xca/0x220
    [<ffffffffa03eddd6>] br_vlan_add+0x66/0xe0 [bridge]
    [<ffffffffa03e543c>] br_setlink+0x2dc/0x340 [bridge]
    [<ffffffff8150e481>] rtnl_bridge_setlink+0x101/0x200
    [<ffffffff8150d9d9>] rtnetlink_rcv_msg+0x99/0x260
    [<ffffffff81528679>] netlink_rcv_skb+0xa9/0xc0
    [<ffffffff8150d938>] rtnetlink_rcv+0x28/0x30
    [<ffffffff81527ccd>] netlink_unicast+0xdd/0x190
    [<ffffffff8152807f>] netlink_sendmsg+0x2ff/0x740
    [<ffffffff814e8368>] sock_sendmsg+0x88/0xc0
    [<ffffffff814e8ac8>] ___sys_sendmsg.part.14+0x298/0x2b0
    [<ffffffff814e91de>] __sys_sendmsg+0x4e/0x90
    [<ffffffff814e922e>] SyS_sendmsg+0xe/0x10
    [<ffffffff81601669>] system_call_fastpath+0x16/0x1b
    [<ffffffffffffffff>] 0xffffffffffffffff

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>