workqueue: fix null-ptr-deref on __alloc_workqueue() error

wq->lockdep_map is set only after __alloc_workqueue()
successfully returns. However, on its error path
__alloc_workqueue() may call destroy_workqueue() which
expects wq->lockdep_map to be already set, which results
in a null-ptr-deref in touch_wq_lockdep_map().

Add a simple NULL-check to touch_wq_lockdep_map().

Oops: general protection fault, probably for non-canonical address
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:__lock_acquire+0x81/0x7800
[..]
Call Trace:
 <TASK>
 ? __die_body+0x66/0xb0
 ? die_addr+0xb2/0xe0
 ? exc_general_protection+0x300/0x470
 ? asm_exc_general_protection+0x22/0x30
 ? __lock_acquire+0x81/0x7800
 ? mark_lock+0x94/0x330
 ? __lock_acquire+0x12fd/0x7800
 ? __lock_acquire+0x3439/0x7800
 lock_acquire+0x14c/0x3e0
 ? __flush_workqueue+0x167/0x13a0
 ? __init_swait_queue_head+0xaf/0x150
 ? __flush_workqueue+0x167/0x13a0
 __flush_workqueue+0x17d/0x13a0
 ? __flush_workqueue+0x167/0x13a0
 ? lock_release+0x50f/0x830
 ? drain_workqueue+0x94/0x300
 drain_workqueue+0xe3/0x300
 destroy_workqueue+0xac/0xc40
 ? workqueue_sysfs_register+0x159/0x2f0
 __alloc_workqueue+0x1506/0x1760
 alloc_workqueue+0x61/0x150
...

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: sergey-senozhatsky

kernel/workqueue.c

@@ -3871,6 +3871,9 @@ static bool flush_workqueue_prep_pwqs(struct workqueue_struct *wq,
 static void touch_wq_lockdep_map(struct workqueue_struct *wq)
 {
 #ifdef CONFIG_LOCKDEP
+	if (unlikely(!wq->lockdep_map))
+		return;
+
 	if (wq->flags & WQ_BH)
 		local_bh_disable();