wifi: mac80211: fix initialization of rx->link and rx->link_sta

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

wifi: mac80211: fix initialization of rx->link and rx->link_sta

There are some codepaths that do not initialize rx->link_sta properly. This
causes a crash in places which assume that rx->link_sta is valid if rx->sta
is valid.
One known instance is triggered by __ieee80211_rx_h_amsdu being called from
fast-rx. It results in a crash like this one:

BUG: kernel NULL pointer dereference, address: 00000000000000a8
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP PTI
CPU: 1 PID: 506 Comm: mt76-usb-rx phy Tainted: G E 6.1.0-debian64x+1.7 #3
Hardware name: ZOTAC ZBOX-ID92/ZBOX-IQ01/ZBOX-ID92/ZBOX-IQ01, BIOS B220P007 05/21/2014
RIP: 0010:ieee80211_deliver_skb+0x62/0x1f0 [mac80211]
Code: 00 48 89 04 24 e8 9e a7 c3 df 89 c0 48 03 1c c5 a0 ea 39 a1 4c 01 6b 08 48 ff 03 48
83 7d 28 00 74 11 48 8b 45 30 48 63 55 44 <48> 83 84 d0 a8 00 00 00 01 41 8b 86 c0
11 00 00 8d 50 fd 83 fa 01
RSP: 0018:ffff999040803b10 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffffb9903f496480 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffff999040803ce0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8d21828ac900
R13: 000000000000004a R14: ffff8d2198ed89c0 R15: ffff8d2198ed8000
FS: 0000000000000000(0000) GS:ffff8d24afe80000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000a8 CR3: 0000000429810002 CR4: 00000000001706e0
Call Trace:
<TASK>
__ieee80211_rx_h_amsdu+0x1b5/0x240 [mac80211]
? ieee80211_prepare_and_rx_handle+0xcdd/0x1320 [mac80211]
? __local_bh_enable_ip+0x3b/0xa0
ieee80211_prepare_and_rx_handle+0xcdd/0x1320 [mac80211]
? prepare_transfer+0x109/0x1a0 [xhci_hcd]
ieee80211_rx_list+0xa80/0xda0 [mac80211]
mt76_rx_complete+0x207/0x2e0 [mt76]
mt76_rx_poll_complete+0x357/0x5a0 [mt76]
mt76u_rx_worker+0x4f5/0x600 [mt76_usb]
? mt76_get_min_avg_rssi+0x140/0x140 [mt76]
__mt76_worker_fn+0x50/0x80 [mt76]
kthread+0xed/0x120
? kthread_complete_and_exit+0x20/0x20
ret_from_fork+0x22/0x30

Since the initialization of rx->link and rx->link_sta is rather convoluted
and duplicated in many places, clean it up by using a helper function to
set it.

[remove unnecessary rx->sta->sta.mlo check]