mlxsw: spectrum_router: Fix neighbour use-after-free

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

mlxsw: spectrum_router: Fix neighbour use-after-free

We sometimes observe use-after-free when dereferencing a neighbour [1].
The problem seems to be that the driver stores a pointer to the
neighbour, but without holding a reference on it. A reference is only
taken when the neighbour is used by a nexthop.

Fix by simplifying the reference counting scheme. Always take a
reference when storing a neighbour pointer in a neighbour entry. Avoid
taking a referencing when the neighbour is used by a nexthop as the
neighbour entry associated with the nexthop already holds a reference.

Tested by running the test that uncovered the problem over 300 times.
Without this patch the problem was reproduced after a handful of
iterations.

[1]
BUG: KASAN: slab-use-after-free in mlxsw_sp_neigh_entry_update+0x2d4/0x310
Read of size 8 at addr ffff88817f8e3420 by task ip/3929

CPU: 3 UID: 0 PID: 3929 Comm: ip Not tainted 6.18.0-rc4-virtme-g36b21a067510 #3 PREEMPT(full)
Hardware name: Nvidia SN5600/VMOD0013, BIOS 5.13 05/31/2023
Call Trace:
<TASK>
dump_stack_lvl+0x6f/0xa0
print_address_description.constprop.0+0x6e/0x300
print_report+0xfc/0x1fb
kasan_report+0xe4/0x110
mlxsw_sp_neigh_entry_update+0x2d4/0x310
mlxsw_sp_router_rif_gone_sync+0x35f/0x510
mlxsw_sp_rif_destroy+0x1ea/0x730
mlxsw_sp_inetaddr_port_vlan_event+0xa1/0x1b0
__mlxsw_sp_inetaddr_lag_event+0xcc/0x130
__mlxsw_sp_inetaddr_event+0xf5/0x3c0
mlxsw_sp_router_netdevice_event+0x1015/0x1580
notifier_call_chain+0xcc/0x150
call_netdevice_notifiers_info+0x7e/0x100
__netdev_upper_dev_unlink+0x10b/0x210
netdev_upper_dev_unlink+0x79/0xa0
vrf_del_slave+0x18/0x50
do_set_master+0x146/0x7d0
do_setlink.isra.0+0x9a0/0x2880
rtnl_newlink+0x637/0xb20
rtnetlink_rcv_msg+0x6fe/0xb90
netlink_rcv_skb+0x123/0x380
netlink_unicast+0x4a3/0x770
netlink_sendmsg+0x75b/0xc90
__sock_sendmsg+0xbe/0x160
____sys_sendmsg+0x5b2/0x7d0
___sys_sendmsg+0xfd/0x180
__sys_sendmsg+0x124/0x1c0
do_syscall_64+0xbb/0xfd0
entry_SYSCALL_64_after_hwframe+0x4b/0x53
[...]

Allocated by task 109:
kasan_save_stack+0x30/0x50
kasan_save_track+0x14/0x30
__kasan_kmalloc+0x7b/0x90
__kmalloc_noprof+0x2c1/0x790
neigh_alloc+0x6af/0x8f0
___neigh_create+0x63/0xe90
mlxsw_sp_nexthop_neigh_init+0x430/0x7e0
mlxsw_sp_nexthop_type_init+0x212/0x960
mlxsw_sp_nexthop6_group_info_init.constprop.0+0x81f/0x1280
mlxsw_sp_nexthop6_group_get+0x392/0x6a0
mlxsw_sp_fib6_entry_create+0x46a/0xfd0
mlxsw_sp_router_fib6_replace+0x1ed/0x5f0
mlxsw_sp_router_fib6_event_work+0x10a/0x2a0
process_one_work+0xd57/0x1390
worker_thread+0x4d6/0xd40
kthread+0x355/0x5b0
ret_from_fork+0x1d4/0x270
ret_from_fork_asm+0x11/0x20

Freed by task 154:
kasan_save_stack+0x30/0x50
kasan_save_track+0x14/0x30
__kasan_save_free_info+0x3b/0x60
__kasan_slab_free+0x43/0x70
kmem_cache_free_bulk.part.0+0x1eb/0x5e0
kvfree_rcu_bulk+0x1f2/0x260
kfree_rcu_work+0x130/0x1b0
process_one_work+0xd57/0x1390
worker_thread+0x4d6/0xd40
kthread+0x355/0x5b0
ret_from_fork+0x1d4/0x270
ret_from_fork_asm+0x11/0x20

Last potentially related work creation:
kasan_save_stack+0x30/0x50
kasan_record_aux_stack+0x8c/0xa0
kvfree_call_rcu+0x93/0x5b0
mlxsw_sp_router_neigh_event_work+0x67d/0x860
process_one_work+0xd57/0x1390
worker_thread+0x4d6/0xd40
kthread+0x355/0x5b0
ret_from_fork+0x1d4/0x270
ret_from_fork_asm+0x11/0x20