WARNING: lock held when returning to user space!

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
38c23685b273cfb4ccf31a199feccce3bdcb5d83 (Fri Apr 6 04:29:35 2018 +0000)
Merge tag 'armsoc-drivers' of  
git://git.kernel.org/pub/scm/linux/kernel/git/arm/arm-soc
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=31e8daa8b3fc129e75f2

So far this crash happened 9 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6407930337296384
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=4942413340606464
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=4764483918495744
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-5813481738265533882
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

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Reported-by: syzbot+31e8daa8b3fc129e75f2@xxxxxxxxxxxxxxxxxxxxxxxxx
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================================================
WARNING: lock held when returning to user space!
4.16.0+ #3 Not tainted
------------------------------------------------
syzkaller433111/4462 is leaving the kernel with locks still held!
1 lock held by syzkaller433111/4462:
 #0: 0000000003a06fae (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x1ec0  
drivers/block/loop.c:1363


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