gtp: Suppress list corruption splat in gtp_net_exit_batch_rtnl().

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

gtp: Suppress list corruption splat in gtp_net_exit_batch_rtnl().

Brad Spengler reported the list_del() corruption splat in
gtp_net_exit_batch_rtnl(). [0]

Commit eb28fd76c0a0 ("gtp: Destroy device along with udp socket's netns
dismantle.") added the for_each_netdev() loop in gtp_net_exit_batch_rtnl()
to destroy devices in each netns as done in geneve and ip tunnels.

However, this could trigger ->dellink() twice for the same device during
->exit_batch_rtnl().

Say we have two netns A & B and gtp device B that resides in netns B but
whose UDP socket is in netns A.

1. cleanup_net() processes netns A and then B.

2. gtp_net_exit_batch_rtnl() finds the device B while iterating
netns A's gn->gtp_dev_list and calls ->dellink().

[ device B is not yet unlinked from netns B
as unregister_netdevice_many() has not been called. ]

3. gtp_net_exit_batch_rtnl() finds the device B while iterating
netns B's for_each_netdev() and calls ->dellink().

gtp_dellink() cleans up the device's hash table, unlinks the dev from
gn->gtp_dev_list, and calls unregister_netdevice_queue().

Basically, calling gtp_dellink() multiple times is fine unless
CONFIG_DEBUG_LIST is enabled.

Let's remove for_each_netdev() in gtp_net_exit_batch_rtnl() and
delegate the destruction to default_device_exit_batch() as done
in bareudp.

[0]:
list_del corruption, ffff8880aaa62c00->next (autoslab_size_M_dev_P_net_core_dev_11127_8_1328_8_S_4096_A_64_n_139+0xc00/0x1000 [slab object]) is LIST_POISON1 (ffffffffffffff02) (prev is 0xffffffffffffff04)
kernel BUG at lib/list_debug.c:58!
Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 UID: 0 PID: 1804 Comm: kworker/u8:7 Tainted: G T 6.12.13-grsec-full-20250211091339 #1
Tainted: [T]=RANDSTRUCT
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Workqueue: netns cleanup_net
RIP: 0010:[<ffffffff84947381>] __list_del_entry_valid_or_report+0x141/0x200 lib/list_debug.c:58
Code: c2 76 91 31 c0 e8 9f b1 f7 fc 0f 0b 4d 89 f0 48 c7 c1 02 ff ff ff 48 89 ea 48 89 ee 48 c7 c7 e0 c2 76 91 31 c0 e8 7f b1 f7 fc <0f> 0b 4d 89 e8 48 c7 c1 04 ff ff ff 48 89 ea 48 89 ee 48 c7 c7 60
RSP: 0018:fffffe8040b4fbd0 EFLAGS: 00010283
RAX: 00000000000000cc RBX: dffffc0000000000 RCX: ffffffff818c4054
RDX: ffffffff84947381 RSI: ffffffff818d1512 RDI: 0000000000000000
RBP: ffff8880aaa62c00 R08: 0000000000000001 R09: fffffbd008169f32
R10: fffffe8040b4f997 R11: 0000000000000001 R12: a1988d84f24943e4
R13: ffffffffffffff02 R14: ffffffffffffff04 R15: ffff8880aaa62c08
RBX: kasan shadow of 0x0
RCX: __wake_up_klogd.part.0+0x74/0xe0 kernel/printk/printk.c:4554
RDX: __list_del_entry_valid_or_report+0x141/0x200 lib/list_debug.c:58
RSI: vprintk+0x72/0x100 kernel/printk/printk_safe.c:71
RBP: autoslab_size_M_dev_P_net_core_dev_11127_8_1328_8_S_4096_A_64_n_139+0xc00/0x1000 [slab object]
RSP: process kstack fffffe8040b4fbd0+0x7bd0/0x8000 [kworker/u8:7+netns 1804 ]
R09: kasan shadow of process kstack fffffe8040b4f990+0x7990/0x8000 [kworker/u8:7+netns 1804 ]
R10: process kstack fffffe8040b4f997+0x7997/0x8000 [kworker/u8:7+netns 1804 ]
R15: autoslab_size_M_dev_P_net_core_dev_11127_8_1328_8_S_4096_A_64_n_139+0xc08/0x1000 [slab object]
FS: 0000000000000000(0000) GS:ffff888116000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000748f5372c000 CR3: 0000000015408000 CR4: 00000000003406f0 shadow CR4: 00000000003406f0
Stack:
0000000000000000 ffffffff8a0c35e7 ffffffff8a0c3603 ffff8880aaa62c00
ffff8880aaa62c00 0000000000000004 ffff88811145311c 0000000000000005
0000000000000001 ffff8880aaa62000 fffffe8040b4fd40 ffffffff8a0c360d
Call Trace:
<TASK>
[<ffffffff8a0c360d>] __list_del_entry_valid include/linux/list.h:131 [inline] fffffe8040b4fc28
[<ffffffff8a0c360d>] __list_del_entry include/linux/list.h:248 [inline] fffffe8040b4fc28
[<ffffffff8a0c360d>] list_del include/linux/list.h:262 [inl
---truncated---