net: marvell: prestera: fix incorrect structure access

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

net: marvell: prestera: fix incorrect structure access

In line:
upper = info->upper_dev;
We access upper_dev field, which is related only for particular events
(e.g. event == NETDEV_CHANGEUPPER). So, this line cause invalid memory
access for another events,
when ptr is not netdev_notifier_changeupper_info.

The KASAN logs are as follows:

[ 30.123165] BUG: KASAN: stack-out-of-bounds in prestera_netdev_port_event.constprop.0+0x68/0x538 [prestera]
[ 30.133336] Read of size 8 at addr ffff80000cf772b0 by task udevd/778
[ 30.139866]
[ 30.141398] CPU: 0 PID: 778 Comm: udevd Not tainted 5.16.0-rc3 #6
[ 30.147588] Hardware name: DNI AmazonGo1 A7040 board (DT)
[ 30.153056] Call trace:
[ 30.155547] dump_backtrace+0x0/0x2c0
[ 30.159320] show_stack+0x18/0x30
[ 30.162729] dump_stack_lvl+0x68/0x84
[ 30.166491] print_address_description.constprop.0+0x74/0x2b8
[ 30.172346] kasan_report+0x1e8/0x250
[ 30.176102] __asan_load8+0x98/0xe0
[ 30.179682] prestera_netdev_port_event.constprop.0+0x68/0x538 [prestera]
[ 30.186847] prestera_netdev_event_handler+0x1b4/0x1c0 [prestera]
[ 30.193313] raw_notifier_call_chain+0x74/0xa0
[ 30.197860] call_netdevice_notifiers_info+0x68/0xc0
[ 30.202924] register_netdevice+0x3cc/0x760
[ 30.207190] register_netdev+0x24/0x50
[ 30.211015] prestera_device_register+0x8a0/0xba0 [prestera]