null_blk: disk nullb1 created

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WARNING: lock held when returning to user space!
syzkaller #0 Not tainted
------------------------------------------------
syz.0.0/5327 is leaving the kernel with locks still held!
locks held by syz.0.0/5327: 2, last CPU#0:
 #0: ffff888034c3d430 (&q->q_usage_counter(io)#40){++++}-{0:0}, at: blk_mq_freeze_queue include/linux/blk-mq.h:956 [inline]
 #0: ffff888034c3d430 (&q->q_usage_counter(io)#40){++++}-{0:0}, at: blk_throtl_init+0x27b/0x420 block/blk-throttle.c:1332
 #1: ffff888034c3d468 (&q->q_usage_counter(queue)#22){+.+.}-{0:0}, at: blk_mq_freeze_queue include/linux/blk-mq.h:956 [inline]
 #1: ffff888034c3d468 (&q->q_usage_counter(queue)#22){+.+.}-{0:0}, at: blk_throtl_init+0x27b/0x420 block/blk-throttle.c:1332