ieee80211_tx_ba_session_handle_start() may get NULL for sdata when a
deauthentication is ongoing.

Here a trace triggering the race with the hostapd test
multi_ap_fronthaul_on_ap:

(gdb) list *drv_ampdu_action+0x46
0x8b16 is in drv_ampdu_action (net/mac80211/driver-ops.c:396).
391             int ret = -EOPNOTSUPP;
392
393             might_sleep();
394
395             sdata = get_bss_sdata(sdata);
396             if (!check_sdata_in_driver(sdata))
397                     return -EIO;
398
399             trace_drv_ampdu_action(local, sdata, params);
400

wlan0: moving STA 02:00:00:00:03:00 to state 3
wlan0: associated
wlan0: deauthenticating from 02:00:00:00:03:00 by local choice (Reason: 3=DEAUTH_LEAVING)
wlan3.sta1: Open BA session requested for 02:00:00:00:00:00 tid 0
wlan3.sta1: dropped frame to 02:00:00:00:00:00 (unauthorized port)
wlan0: moving STA 02:00:00:00:03:00 to state 2
wlan0: moving STA 02:00:00:00:03:00 to state 1
wlan0: Removed STA 02:00:00:00:03:00
wlan0: Destroyed STA 02:00:00:00:03:00
BUG: unable to handle page fault for address: fffffffffffffb48
PGD 11814067 P4D 11814067 PUD 11816067 PMD 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 2 PID: 133397 Comm: kworker/u16:1 Tainted: G        W          6.1.0-rc8-wt+ #59
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.0-20220807_005459-localhost 04/01/2014
Workqueue: phy3 ieee80211_ba_session_work [mac80211]
RIP: 0010:drv_ampdu_action+0x46/0x280 [mac80211]
Code: 53 48 89 f3 be 89 01 00 00 e8 d6 43 bf ef e8 21 46 81 f0 83 bb a0 1b 00 00 04 75 0e 48 8b 9b 28 0d 00 00 48 81 eb 10 0e 00 00 <8b> 93 58 09 00 00 f6 c2 20 0f 84 3b 01 00 00 8b 05 dd 1c 0f 00 85
RSP: 0018:ffffc900025ebd20 EFLAGS: 00010287
RAX: 0000000000000000 RBX: fffffffffffff1f0 RCX: ffff888102228240
RDX: 0000000080000000 RSI: ffffffff918c5de0 RDI: ffff888102228b40
RBP: ffffc900025ebd40 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000001 R11: 0000000000000000 R12: ffff888118c18ec0
R13: 0000000000000000 R14: ffffc900025ebd60 R15: ffff888018b7efb8
FS:  0000000000000000(0000) GS:ffff88817a600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: fffffffffffffb48 CR3: 0000000105228006 CR4: 0000000000170ee0
Call Trace:
 <TASK>
 ieee80211_tx_ba_session_handle_start+0xd0/0x190 [mac80211]
 ieee80211_ba_session_work+0xff/0x2e0 [mac80211]
 process_one_work+0x29f/0x620
 worker_thread+0x4d/0x3d0
 ? process_one_work+0x620/0x620
 kthread+0xfb/0x120
 ? kthread_complete_and_exit+0x20/0x20
 ret_from_fork+0x22/0x30
 </TASK>
Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Link: https://lore.kernel.org/r/20221230121850.218810-2-alexander@wetzel-home.de
Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>