bridge: Fix possible use-after-free when deleting bridge port

When a bridge port is being deleted, do not dereference it later in
br_vlan_port_event() as it can result in a use-after-free [1] if the RCU
callback was executed before invoking the function.

[1]
[  129.638551] ==================================================================
[  129.646904] BUG: KASAN: use-after-free in br_vlan_port_event+0x53c/0x5fd
[  129.654406] Read of size 8 at addr ffff8881e4aa1ae8 by task ip/483
[  129.663008] CPU: 0 PID: 483 Comm: ip Not tainted 5.1.0-rc5-custom-02265-ga946bd73daac #1383
[  129.672359] Hardware name: Mellanox Technologies Ltd. MSN2100-CB2FO/SA001017, BIOS 5.6.5 06/07/2016
[  129.682484] Call Trace:
[  129.685242]  dump_stack+0xa9/0x10e
[  129.689068]  print_address_description.cold.2+0x9/0x25e
[  129.694930]  kasan_report.cold.3+0x78/0x9d
[  129.704420]  br_vlan_port_event+0x53c/0x5fd
[  129.728300]  br_device_event+0x2c7/0x7a0
[  129.741505]  notifier_call_chain+0xb5/0x1c0
[  129.746202]  rollback_registered_many+0x895/0xe90
[  129.793119]  unregister_netdevice_many+0x48/0x210
[  129.803384]  rtnl_delete_link+0xe1/0x140
[  129.815906]  rtnl_dellink+0x2a3/0x820
[  129.844166]  rtnetlink_rcv_msg+0x397/0x910
[  129.868517]  netlink_rcv_skb+0x137/0x3a0
[  129.882013]  netlink_unicast+0x49b/0x660
[  129.900019]  netlink_sendmsg+0x755/0xc90
[  129.915758]  ___sys_sendmsg+0x761/0x8e0
[  129.966315]  __sys_sendmsg+0xf0/0x1c0
[  129.988918]  do_syscall_64+0xa4/0x470
[  129.993032]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  129.998696] RIP: 0033:0x7ff578104b58
...
[  130.073811] Allocated by task 479:
[  130.077633]  __kasan_kmalloc.constprop.5+0xc1/0xd0
[  130.083008]  kmem_cache_alloc_trace+0x152/0x320
[  130.088090]  br_add_if+0x39c/0x1580
[  130.092005]  do_set_master+0x1aa/0x210
[  130.096211]  do_setlink+0x985/0x3100
[  130.100224]  __rtnl_newlink+0xc52/0x1380
[  130.104625]  rtnl_newlink+0x6b/0xa0
[  130.108541]  rtnetlink_rcv_msg+0x397/0x910
[  130.113136]  netlink_rcv_skb+0x137/0x3a0
[  130.117538]  netlink_unicast+0x49b/0x660
[  130.121939]  netlink_sendmsg+0x755/0xc90
[  130.126340]  ___sys_sendmsg+0x761/0x8e0
[  130.130645]  __sys_sendmsg+0xf0/0x1c0
[  130.134753]  do_syscall_64+0xa4/0x470
[  130.138864]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

[  130.146195] Freed by task 0:
[  130.149421]  __kasan_slab_free+0x125/0x170
[  130.154016]  kfree+0xf3/0x310
[  130.157349]  kobject_put+0x1a8/0x4c0
[  130.161363]  rcu_core+0x859/0x19b0
[  130.165175]  __do_softirq+0x250/0xa26
[  130.170956] The buggy address belongs to the object at ffff8881e4aa1ae8
                which belongs to the cache kmalloc-1k of size 1024
[  130.184972] The buggy address is located 0 bytes inside of
                1024-byte region [ffff8881e4aa1ae8, ffff8881e4aa1ee8)

Fixes: 9c0ec2e7 ("bridge: support binding vlan dev link state to vlan member bridge ports")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Cc: Mike Manning <mmanning@vyatta.att-mail.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Mike Manning <mmanning@vyatta.att-mail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>