gtp: Destroy device along with udp socket's netns dismantle.

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

gtp: Destroy device along with udp socket's netns dismantle.

gtp_newlink() links the device to a list in dev_net(dev) instead of
src_net, where a udp tunnel socket is created.

Even when src_net is removed, the device stays alive on dev_net(dev).
Then, removing src_net triggers the splat below. [0]

In this example, gtp0 is created in ns2, and the udp socket is created
in ns1.

ip netns add ns1
ip netns add ns2
ip -n ns1 link add netns ns2 name gtp0 type gtp role sgsn
ip netns del ns1

Let's link the device to the socket's netns instead.

Now, gtp_net_exit_batch_rtnl() needs another netdev iteration to remove
all gtp devices in the netns.

[0]:
ref_tracker: net notrefcnt@000000003d6e7d05 has 1/2 users at
sk_alloc (./include/net/net_namespace.h:345 net/core/sock.c:2236)
inet_create (net/ipv4/af_inet.c:326 net/ipv4/af_inet.c:252)
__sock_create (net/socket.c:1558)
udp_sock_create4 (net/ipv4/udp_tunnel_core.c:18)
gtp_create_sock (./include/net/udp_tunnel.h:59 drivers/net/gtp.c:1423)
gtp_create_sockets (drivers/net/gtp.c:1447)
gtp_newlink (drivers/net/gtp.c:1507)
rtnl_newlink (net/core/rtnetlink.c:3786 net/core/rtnetlink.c:3897 net/core/rtnetlink.c:4012)
rtnetlink_rcv_msg (net/core/rtnetlink.c:6922)
netlink_rcv_skb (net/netlink/af_netlink.c:2542)
netlink_unicast (net/netlink/af_netlink.c:1321 net/netlink/af_netlink.c:1347)
netlink_sendmsg (net/netlink/af_netlink.c:1891)
____sys_sendmsg (net/socket.c:711 net/socket.c:726 net/socket.c:2583)
___sys_sendmsg (net/socket.c:2639)
__sys_sendmsg (net/socket.c:2669)
do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)

WARNING: CPU: 1 PID: 60 at lib/ref_tracker.c:179 ref_tracker_dir_exit (lib/ref_tracker.c:179)
Modules linked in:
CPU: 1 UID: 0 PID: 60 Comm: kworker/u16:2 Not tainted 6.13.0-rc5-00147-g4c1224501e9d #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Workqueue: netns cleanup_net
RIP: 0010:ref_tracker_dir_exit (lib/ref_tracker.c:179)
Code: 00 00 00 fc ff df 4d 8b 26 49 bd 00 01 00 00 00 00 ad de 4c 39 f5 0f 85 df 00 00 00 48 8b 74 24 08 48 89 df e8 a5 cc 12 02 90 <0f> 0b 90 48 8d 6b 44 be 04 00 00 00 48 89 ef e8 80 de 67 ff 48 89
RSP: 0018:ff11000009a07b60 EFLAGS: 00010286
RAX: 0000000000002bd3 RBX: ff1100000f4e1aa0 RCX: 1ffffffff0e40ac6
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8423ee3c
RBP: ff1100000f4e1af0 R08: 0000000000000001 R09: fffffbfff0e395ae
R10: 0000000000000001 R11: 0000000000036001 R12: ff1100000f4e1af0
R13: dead000000000100 R14: ff1100000f4e1af0 R15: dffffc0000000000
FS: 0000000000000000(0000) GS:ff1100006ce80000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f9b2464bd98 CR3: 0000000005286005 CR4: 0000000000771ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<TASK>
? __warn (kernel/panic.c:748)
? ref_tracker_dir_exit (lib/ref_tracker.c:179)
? report_bug (lib/bug.c:201 lib/bug.c:219)
? handle_bug (arch/x86/kernel/traps.c:285)
? exc_invalid_op (arch/x86/kernel/traps.c:309 (discriminator 1))
? asm_exc_invalid_op (./arch/x86/include/asm/idtentry.h:621)
? _raw_spin_unlock_irqrestore (./arch/x86/include/asm/irqflags.h:42 ./arch/x86/include/asm/irqflags.h:97 ./arch/x86/include/asm/irqflags.h:155 ./include/linux/spinlock_api_smp.h:151 kernel/locking/spinlock.c:194)
? ref_tracker_dir_exit (lib/ref_tracker.c:179)
? __pfx_ref_tracker_dir_exit (lib/ref_tracker.c:158)
? kfree (mm/slub.c:4613 mm/slub.c:4761)
net_free (net/core/net_namespace.c:476 net/core/net_namespace.c:467)
cleanup_net (net/core/net_namespace.c:664 (discriminator 3))
process_one_work (kernel/workqueue.c:3229)
worker_thread (kernel/workqueue.c:3304 kernel/workqueue.c:3391
---truncated---