wifi: ath12k: Avoid accessing uninitialized arvif->ar during beacon miss

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

wifi: ath12k: Avoid accessing uninitialized arvif->ar during beacon miss

During beacon miss handling, ath12k driver iterates over active virtual
interfaces (vifs) and attempts to access the radio object (ar) via
arvif->deflink->ar.

However, after commit aa80f12f3bed ("wifi: ath12k: defer vdev creation for
MLO"), arvif is linked to a radio only after vdev creation, typically when
a channel is assigned or a scan is requested.
For P2P capable devices, a default P2P interface is created by
wpa_supplicant along with regular station interfaces, these serve as dummy
interfaces for P2P-capable stations, lack an associated netdev and initiate
frequent scans to discover neighbor p2p devices. When a scan is initiated
on such P2P vifs, driver selects destination radio (ar) based on scan
frequency, creates a scan vdev, and attaches arvif to the radio. Once the
scan completes or is aborted, the scan vdev is deleted, detaching arvif
from the radio and leaving arvif->ar uninitialized.

While handling beacon miss for station interfaces, P2P interface is also
encountered in the vif iteration and ath12k_mac_handle_beacon_miss_iter()
tries to dereference the uninitialized arvif->deflink->ar.

Fix this by verifying that vdev is created for the arvif before accessing
its ar during beacon miss handling and similar vif iterator callbacks.

==========================================================================
wlp6s0: detected beacon loss from AP (missed 7 beacons) - probing
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]

CPU: 5 UID: 0 PID: 0 Comm: swapper/5 Not tainted 6.16.0-rc1-wt-ath+ #2 PREEMPT(full)
RIP: 0010:ath12k_mac_handle_beacon_miss_iter+0xb5/0x1a0 [ath12k]
Call Trace:
__iterate_interfaces+0x11a/0x410 [mac80211]
ieee80211_iterate_active_interfaces_atomic+0x61/0x140 [mac80211]
ath12k_mac_handle_beacon_miss+0xa1/0xf0 [ath12k]
ath12k_roam_event+0x393/0x560 [ath12k]
ath12k_wmi_op_rx+0x1486/0x28c0 [ath12k]
ath12k_htc_process_trailer.isra.0+0x2fb/0x620 [ath12k]
ath12k_htc_rx_completion_handler+0x448/0x830 [ath12k]
ath12k_ce_recv_process_cb+0x549/0x9e0 [ath12k]
ath12k_ce_per_engine_service+0xbe/0xf0 [ath12k]
ath12k_pci_ce_workqueue+0x69/0x120 [ath12k]
process_one_work+0xe3a/0x1430

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.4.1-00199-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.1.c5-00284.1-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3