net: core: use list_del_init() instead of list_del() in netdev_run_todo()

dev->unlink_list is reused unless dev is deleted.
So, list_del() should not be used.
Due to using list_del(), dev->unlink_list can't be reused so that
dev->nested_level update logic doesn't work.
In order to fix this bug, list_del_init() should be used instead
of list_del().

Test commands:
ip link add bond0 type bond
ip link add bond1 type bond
ip link set bond0 master bond1
ip link set bond0 nomaster
ip link set bond1 master bond0
ip link set bond1 nomaster

Splat looks like:
[ 255.750458][ T1030] ============================================
[ 255.751967][ T1030] WARNING: possible recursive locking detected
[ 255.753435][ T1030] 5.9.0-rc8+ #772 Not tainted
[ 255.754553][ T1030] --------------------------------------------
[ 255.756047][ T1030] ip/1030 is trying to acquire lock:
[ 255.757304][ T1030] ffff88811782a280 (&dev_addr_list_lock_key/1){+...}-{2:2}, at: dev_mc_sync_multiple+0xc2/0x150
[ 255.760056][ T1030]
[ 255.760056][ T1030] but task is already holding lock:
[ 255.761862][ T1030] ffff88811130a280 (&dev_addr_list_lock_key/1){+...}-{2:2}, at: bond_enslave+0x3d4d/0x43e0 [bonding]
[ 255.764581][ T1030]
[ 255.764581][ T1030] other info that might help us debug this:
[ 255.766645][ T1030] Possible unsafe locking scenario:
[ 255.766645][ T1030]
[ 255.768566][ T1030] CPU0
[ 255.769415][ T1030] ----
[ 255.770259][ T1030] lock(&dev_addr_list_lock_key/1);
[ 255.771629][ T1030] lock(&dev_addr_list_lock_key/1);
[ 255.772994][ T1030]
[ 255.772994][ T1030] *** DEADLOCK ***
[ 255.772994][ T1030]
[ 255.775091][ T1030] May be due to missing lock nesting notation
[ 255.775091][ T1030]
[ 255.777182][ T1030] 2 locks held by ip/1030:
[ 255.778299][ T1030] #0: ffffffffb1f63250 (rtnl_mutex){+.+.}-{3:3}, at: rtnetlink_rcv_msg+0x2e4/0x8b0
[ 255.780600][ T1030] #1: ffff88811130a280 (&dev_addr_list_lock_key/1){+...}-{2:2}, at: bond_enslave+0x3d4d/0x43e0 [bonding]
[ 255.783411][ T1030]
[ 255.783411][ T1030] stack backtrace:
[ 255.784874][ T1030] CPU: 7 PID: 1030 Comm: ip Not tainted 5.9.0-rc8+ #772
[ 255.786595][ T1030] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 255.789030][ T1030] Call Trace:
[ 255.789850][ T1030] dump_stack+0x99/0xd0
[ 255.790882][ T1030] __lock_acquire.cold.71+0x166/0x3cc
[ 255.792285][ T1030] ? register_lock_class+0x1a30/0x1a30
[ 255.793619][ T1030] ? rcu_read_lock_sched_held+0x91/0xc0
[ 255.794963][ T1030] ? rcu_read_lock_bh_held+0xa0/0xa0
[ 255.796246][ T1030] lock_acquire+0x1b8/0x850
[ 255.797332][ T1030] ? dev_mc_sync_multiple+0xc2/0x150
[ 255.798624][ T1030] ? bond_enslave+0x3d4d/0x43e0 [bonding]
[ 255.800039][ T1030] ? check_flags+0x50/0x50
[ 255.801143][ T1030] ? lock_contended+0xd80/0xd80
[ 255.802341][ T1030] _raw_spin_lock_nested+0x2e/0x70
[ 255.803592][ T1030] ? dev_mc_sync_multiple+0xc2/0x150
[ 255.804897][ T1030] dev_mc_sync_multiple+0xc2/0x150
[ 255.806168][ T1030] bond_enslave+0x3d58/0x43e0 [bonding]
[ 255.807542][ T1030] ? __lock_acquire+0xe53/0x51b0
[ 255.808824][ T1030] ? bond_update_slave_arr+0xdc0/0xdc0 [bonding]
[ 255.810451][ T1030] ? check_chain_key+0x236/0x5e0
[ 255.811742][ T1030] ? mutex_is_locked+0x13/0x50
[ 255.812910][ T1030] ? rtnl_is_locked+0x11/0x20
[ 255.814061][ T1030] ? netdev_master_upper_dev_get+0xf/0x120
[ 255.815553][ T1030] do_setlink+0x94c/0x3040
[ ... ]

Reported-by: [e-mail]
Fixes: 1fc70edb7d7b ("net: core: add nested_level variable in net_device")
Signed-off-by: Taehee Yoo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/dev.c

···
 		struct net_device *dev = list_first_entry(&unlink_list,
 							  struct net_device,
 							  unlink_list);
-		list_del(&dev->unlink_list);
+		list_del_init(&dev->unlink_list);
 		dev->nested_level = dev->lower_level - 1;
 	}
 #endif