ipv4: fib: Don't dump dying fib_info in fib_leaf_notify().

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

ipv4: fib: Don't dump dying fib_info in fib_leaf_notify().

syzbot reported use-after-free in nsim_fib4_prepare_event(). [0]

The problem is that the following functions call fib_info_hold() /
refcount_inc() while dumping fib_info under RCU, which is unsafe.

* mlxsw_sp_router_fib4_event()
* rocker_router_fib_event()
* nsim_fib4_prepare_event()

refcount_inc_not_zero() must be used, but it would be too late
there.

Let's guarantee the lifetime of fib_info in fib_leaf_notify().

Note that IPv6 does not need the corresponding change since
fib6_table_dump() holds fib6_table.tb6_lock.

[0]:
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: kworker/u8:15/3420
Modules linked in:
CPU: 0 UID: 0 PID: 3420 Comm: kworker/u8:15 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Workqueue: netns cleanup_net
RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 1b f1 22 fd 48 8d 3d 84 cb f1 0a 67 48 0f b9 3a eb 4a e8 08 f1 22 fd 48 8d 3d 81 cb f1 0a <67> 48 0f b9 3a eb 37 e8 f5 f0 22 fd 48 8d 3d 7e cb f1 0a 67 48 0f
RSP: 0018:ffffc9000f2c7270 EFLAGS: 00010293
RAX: ffffffff84a18858 RBX: 0000000000000002 RCX: ffff888032ff9ec0
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8f9353e0
RBP: 0000000000000000 R08: ffff888032ff9ec0 R09: 0000000000000005
R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880570cc000
R13: dffffc0000000000 R14: ffff88802b40563c R15: ffff8880570cc000
FS: 0000000000000000(0000) GS:ffff888126173000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fb1f4d5d000 CR3: 000000006072a000 CR4: 00000000003526f0
Call Trace:
<TASK>
__refcount_add include/linux/refcount.h:-1 [inline]
__refcount_inc include/linux/refcount.h:366 [inline]
refcount_inc include/linux/refcount.h:383 [inline]
fib_info_hold include/net/ip_fib.h:629 [inline]
nsim_fib4_prepare_event drivers/net/netdevsim/fib.c:930 [inline]
nsim_fib_event_schedule_work drivers/net/netdevsim/fib.c:1000 [inline]
nsim_fib_event_nb+0x1055/0x1240 drivers/net/netdevsim/fib.c:1043
call_fib_notifier+0x45/0x80 net/core/fib_notifier.c:25
call_fib_entry_notifier net/ipv4/fib_trie.c:90 [inline]
fib_leaf_notify net/ipv4/fib_trie.c:2176 [inline]
fib_table_notify net/ipv4/fib_trie.c:2194 [inline]
fib_notify+0x36b/0x5e0 net/ipv4/fib_trie.c:2217
fib_net_dump net/core/fib_notifier.c:70 [inline]
register_fib_notifier+0x184/0x360 net/core/fib_notifier.c:108
nsim_fib_create+0x85d/0x9f0 drivers/net/netdevsim/fib.c:1596
nsim_dev_reload_create drivers/net/netdevsim/dev.c:1604 [inline]
nsim_dev_reload_up+0x374/0x7c0 drivers/net/netdevsim/dev.c:1058
devlink_reload+0x501/0x8d0 net/devlink/dev.c:475
devlink_pernet_pre_exit+0x1ff/0x420 net/devlink/core.c:558
ops_pre_exit_list net/core/net_namespace.c:161 [inline]
ops_undo_list+0x187/0x940 net/core/net_namespace.c:234
cleanup_net+0x56e/0x800 net/core/net_namespace.c:702
process_one_work kernel/workqueue.c:3314 [inline]
process_scheduled_works+0xb5d/0x1860 kernel/workqueue.c:3397
worker_thread+0xa53/0xfc0 kernel/workqueue.c:3478
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>