bridge: switchdev: Fix memory leaks when changing VLAN protocol

In the Linux kernel, the following vulnerability has been resolved:

bridge: switchdev: Fix memory leaks when changing VLAN protocol

The bridge driver can offload VLANs to the underlying hardware either
via switchdev or the 8021q driver. When the former is used, the VLAN is
marked in the bridge driver with the 'BR_VLFLAG_ADDED_BY_SWITCHDEV'
private flag.

To avoid the memory leaks mentioned in the cited commit, the bridge
driver will try to delete a VLAN via the 8021q driver if the VLAN is not
marked with the previously mentioned flag.

When the VLAN protocol of the bridge changes, switchdev drivers are
notified via the 'SWITCHDEV_ATTR_ID_BRIDGE_VLAN_PROTOCOL' attribute, but
the 8021q driver is also called to add the existing VLANs with the new
protocol and delete them with the old protocol.

In case the VLANs were offloaded via switchdev, the above behavior is
both redundant and buggy. Redundant because the VLANs are already
programmed in hardware and drivers that support VLAN protocol change
(currently only mlx5) change the protocol upon the switchdev attribute
notification. Buggy because the 8021q driver is called despite these
VLANs being marked with 'BR_VLFLAG_ADDED_BY_SWITCHDEV'. This leads to
memory leaks [1] when the VLANs are deleted.

Fix by not calling the 8021q driver for VLANs that were already
programmed via switchdev.

[1]
unreferenced object 0xffff8881f6771200 (size 256):
comm "ip", pid 446855, jiffies 4298238841 (age 55.240s)
hex dump (first 32 bytes):
00 00 7f 0e 83 88 ff ff 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:
[<00000000012819ac>] vlan_vid_add+0x437/0x750
[<00000000f2281fad>] __br_vlan_set_proto+0x289/0x920
[<000000000632b56f>] br_changelink+0x3d6/0x13f0
[<0000000089d25f04>] __rtnl_newlink+0x8ae/0x14c0
[<00000000f6276baf>] rtnl_newlink+0x5f/0x90
[<00000000746dc902>] rtnetlink_rcv_msg+0x336/0xa00
[<000000001c2241c0>] netlink_rcv_skb+0x11d/0x340
[<0000000010588814>] netlink_unicast+0x438/0x710
[<00000000e1a4cd5c>] netlink_sendmsg+0x788/0xc40
[<00000000e8992d4e>] sock_sendmsg+0xb0/0xe0
[<00000000621b8f91>] ____sys_sendmsg+0x4ff/0x6d0
[<000000000ea26996>] ___sys_sendmsg+0x12e/0x1b0
[<00000000684f7e25>] __sys_sendmsg+0xab/0x130
[<000000004538b104>] do_syscall_64+0x3d/0x90
[<0000000091ed9678>] entry_SYSCALL_64_after_hwframe+0x46/0xb0