team: avoid NETDEV_CHANGEMTU event when unregistering slave

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

team: avoid NETDEV_CHANGEMTU event when unregistering slave

syzbot is reporting

unregister_netdevice: waiting for netdevsim0 to become free. Usage count = 3
ref_tracker: netdev@ffff88807dcf8618 has 1/2 users at
__netdev_tracker_alloc include/linux/netdevice.h:4400 [inline]
netdev_hold include/linux/netdevice.h:4429 [inline]
inetdev_init+0x201/0x4e0 net/ipv4/devinet.c:286
inetdev_event+0x251/0x1610 net/ipv4/devinet.c:1600
notifier_call_chain+0x19d/0x3a0 kernel/notifier.c:85
call_netdevice_notifiers_mtu net/core/dev.c:2318 [inline]
netif_set_mtu_ext+0x5aa/0x800 net/core/dev.c:9886
netif_set_mtu+0xd7/0x1b0 net/core/dev.c:9907
dev_set_mtu+0x126/0x260 net/core/dev_api.c:248
team_port_del+0xb07/0xcb0 drivers/net/team/team_core.c:1333
team_del_slave drivers/net/team/team_core.c:1936 [inline]
team_device_event+0x207/0x5b0 drivers/net/team/team_core.c:2929
notifier_call_chain+0x19d/0x3a0 kernel/notifier.c:85
call_netdevice_notifiers_extack net/core/dev.c:2281 [inline]
call_netdevice_notifiers net/core/dev.c:2295 [inline]
__dev_change_net_namespace+0xcb7/0x2050 net/core/dev.c:12592
do_setlink+0x2ce/0x4590 net/core/rtnetlink.c:3060
rtnl_changelink net/core/rtnetlink.c:3776 [inline]
__rtnl_newlink net/core/rtnetlink.c:3935 [inline]
rtnl_newlink+0x15a9/0x1be0 net/core/rtnetlink.c:4072
rtnetlink_rcv_msg+0x7d5/0xbe0 net/core/rtnetlink.c:6958
netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894

problem. Ido Schimmel found steps to reproduce

ip link add name team1 type team
ip link add name dummy1 mtu 1499 master team1 type dummy
ip netns add ns1
ip link set dev dummy1 netns ns1
ip -n ns1 link del dev dummy1

and also found that the same issue was fixed in the bond driver in
commit f51048c3e07b ("bonding: avoid NETDEV_CHANGEMTU event when
unregistering slave").

Let's do similar thing for the team driver, with commit ad7c7b2172c3 ("net:
hold netdev instance lock during sysfs operations") and commit 303a8487a657
("net: s/__dev_set_mtu/__netif_set_mtu/") also applied.