cfg80211: fix use-after-free in reg_process_hint()

reg_process_hint_country_ie() can free regulatory_request and return
REG_REQ_ALREADY_SET. We shouldn't use regulatory_request after it's
called. KASAN error was observed when this happens.

BUG: KASAN: use-after-free in reg_process_hint+0x839/0x8aa [cfg80211]
Read of size 4 at addr ffff8800c430d434 by task kworker/1:3/89
<snipped>
Workqueue: events reg_todo [cfg80211]
Call Trace:
 dump_stack+0xc1/0x10c
 ? _atomic_dec_and_lock+0x1ad/0x1ad
 ? _raw_spin_lock_irqsave+0xa0/0xd2
 print_address_description+0x86/0x26f
 ? reg_process_hint+0x839/0x8aa [cfg80211]
 kasan_report+0x241/0x29b
 reg_process_hint+0x839/0x8aa [cfg80211]
 reg_todo+0x204/0x5b9 [cfg80211]
 process_one_work+0x55f/0x8d0
 ? worker_detach_from_pool+0x1b5/0x1b5
 ? _raw_spin_unlock_irq+0x65/0xdd
 ? _raw_spin_unlock_irqrestore+0xf3/0xf3
 worker_thread+0x5dd/0x841
 ? kthread_parkme+0x1d/0x1d
 kthread+0x270/0x285
 ? pr_cont_work+0xe3/0xe3
 ? rcu_read_unlock_sched_notrace+0xca/0xca
 ret_from_fork+0x22/0x40

Allocated by task 2718:
 set_track+0x63/0xfa
 __kmalloc+0x119/0x1ac
 regulatory_hint_country_ie+0x38/0x329 [cfg80211]
 __cfg80211_connect_result+0x854/0xadd [cfg80211]
 cfg80211_rx_assoc_resp+0x3bc/0x4f0 [cfg80211]
smsc95xx v1.0.6
 ieee80211_sta_rx_queued_mgmt+0x1803/0x7ed5 [mac80211]
 ieee80211_iface_work+0x411/0x696 [mac80211]
 process_one_work+0x55f/0x8d0
 worker_thread+0x5dd/0x841
 kthread+0x270/0x285
 ret_from_fork+0x22/0x40

Freed by task 89:
 set_track+0x63/0xfa
 kasan_slab_free+0x6a/0x87
 kfree+0xdc/0x470
 reg_process_hint+0x31e/0x8aa [cfg80211]
 reg_todo+0x204/0x5b9 [cfg80211]
 process_one_work+0x55f/0x8d0
 worker_thread+0x5dd/0x841
 kthread+0x270/0x285
 ret_from_fork+0x22/0x40
<snipped>

Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>