We have an interesting memory leak in the bridge when it is being
unregistered and is a slave to a master device which would change the
mac of its slaves on unregister (e.g. bond, team). This is a very
unusual setup but we do end up leaking 1 fdb entry because
dev_set_mac_address() would cause the bridge to insert the new mac address
into its table after all fdbs are flushed, i.e. after dellink() on the
bridge has finished and we call NETDEV_UNREGISTER the bond/team would
release it and will call dev_set_mac_address() to restore its original
address and that in turn will add an fdb in the bridge.
One fix is to check for the bridge dev's reg_state in its
ndo_set_mac_address callback and return an error if the bridge is not in
NETREG_REGISTERED.

Easy steps to reproduce:
 1. add bond in mode != A/B
 2. add any slave to the bond
 3. add bridge dev as a slave to the bond
 4. destroy the bridge device

Trace:
 unreferenced object 0xffff888035c4d080 (size 128):
   comm "ip", pid 4068, jiffies 4296209429 (age 1413.753s)
   hex dump (first 32 bytes):
     41 1d c9 36 80 88 ff ff 00 00 00 00 00 00 00 00  A..6............
     d2 19 c9 5e 3f d7 00 00 00 00 00 00 00 00 00 00  ...^?...........
   backtrace:
     [<00000000ddb525dc>] kmem_cache_alloc+0x155/0x26f
     [<00000000633ff1e0>] fdb_create+0x21/0x486 [bridge]
     [<0000000092b17e9c>] fdb_insert+0x91/0xdc [bridge]
     [<00000000f2a0f0ff>] br_fdb_change_mac_address+0xb3/0x175 [bridge]
     [<000000001de02dbd>] br_stp_change_bridge_id+0xf/0xff [bridge]
     [<00000000ac0e32b1>] br_set_mac_address+0x76/0x99 [bridge]
     [<000000006846a77f>] dev_set_mac_address+0x63/0x9b
     [<00000000d30738fc>] __bond_release_one+0x3f6/0x455 [bonding]
     [<00000000fc7ec01d>] bond_netdev_event+0x2f2/0x400 [bonding]
     [<00000000305d7795>] notifier_call_chain+0x38/0x56
     [<0000000028885d4a>] call_netdevice_notifiers+0x1e/0x23
     [<000000008279477b>] rollback_registered_many+0x353/0x6a4
     [<0000000018ef753a>] unregister_netdevice_many+0x17/0x6f
     [<00000000ba854b7a>] rtnl_delete_link+0x3c/0x43
     [<00000000adf8618d>] rtnl_dellink+0x1dc/0x20a
     [<000000009b6395fd>] rtnetlink_rcv_msg+0x23d/0x268

Fixes: 43598813 ("bridge: add local MAC address to forwarding table (v2)")
Reported-by: syzbot+2add91c08eb181fea1bf@syzkaller.appspotmail.com
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>