mlxsw: spectrum_router: Fix use-after-free when deleting GRE net devices

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

mlxsw: spectrum_router: Fix use-after-free when deleting GRE net devices

The driver only offloads neighbors that are constructed on top of net
devices registered by it or their uppers (which are all Ethernet). The
device supports GRE encapsulation and decapsulation of forwarded
traffic, but the driver will not offload dummy neighbors constructed on
top of GRE net devices as they are not uppers of its net devices:

# ip link add name gre1 up type gre tos inherit local 192.0.2.1 remote 198.51.100.1
# ip neigh add 0.0.0.0 lladdr 0.0.0.0 nud noarp dev gre1
$ ip neigh show dev gre1 nud noarp
0.0.0.0 lladdr 0.0.0.0 NOARP

(Note that the neighbor is not marked with 'offload')

When the driver is reloaded and the existing configuration is replayed,
the driver does not perform the same check regarding existing neighbors
and offloads the previously added one:

# devlink dev reload pci/0000:01:00.0
$ ip neigh show dev gre1 nud noarp
0.0.0.0 lladdr 0.0.0.0 offload NOARP

If the neighbor is later deleted, the driver will ignore the
notification (given the GRE net device is not its upper) and will
therefore keep referencing freed memory, resulting in a use-after-free
[1] when the net device is deleted:

# ip neigh del 0.0.0.0 lladdr 0.0.0.0 dev gre1
# ip link del dev gre1

Fix by skipping neighbor replay if the net device for which the replay
is performed is not our upper.

[1]
BUG: KASAN: slab-use-after-free in mlxsw_sp_neigh_entry_update+0x1ea/0x200
Read of size 8 at addr ffff888155b0e420 by task ip/2282
[...]
Call Trace:
<TASK>
dump_stack_lvl+0x6f/0xa0
print_address_description.constprop.0+0x6f/0x350
print_report+0x108/0x205
kasan_report+0xdf/0x110
mlxsw_sp_neigh_entry_update+0x1ea/0x200
mlxsw_sp_router_rif_gone_sync+0x2a8/0x440
mlxsw_sp_rif_destroy+0x1e9/0x750
mlxsw_sp_netdevice_ipip_ol_event+0x3c9/0xdc0
mlxsw_sp_router_netdevice_event+0x3ac/0x15e0
notifier_call_chain+0xca/0x150
call_netdevice_notifiers_info+0x7f/0x100
unregister_netdevice_many_notify+0xc8c/0x1d90
rtnl_dellink+0x34e/0xa50
rtnetlink_rcv_msg+0x6fb/0xb70
netlink_rcv_skb+0x131/0x360
netlink_unicast+0x426/0x710
netlink_sendmsg+0x75a/0xc20
__sock_sendmsg+0xc1/0x150
____sys_sendmsg+0x5aa/0x7b0
___sys_sendmsg+0xfc/0x180
__sys_sendmsg+0x121/0x1b0
do_syscall_64+0xbb/0x1d0
entry_SYSCALL_64_after_hwframe+0x4b/0x53