general protection fault in io_uring_flush

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Hello,

syzbot found the following issue on:

HEAD commit:    d1d2220c Add linux-next specific files for 20200924
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=150e718d900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=254e028a642027c
dashboard link: https://syzkaller.appspot.com/bug?extid=b64c3e0ed576fc1d70e5
compiler:       gcc (GCC) 10.1.0-syz 20200507
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11f0fe4b900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b64c3e0ed576fc1d70e5@xxxxxxxxxxxxxxxxxxxxxxxxx

general protection fault, probably for non-canonical address 0xdffffc0000000029: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000148-0x000000000000014f]
CPU: 0 PID: 8508 Comm: syz-executor.2 Not tainted 5.9.0-rc6-next-20200924-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:io_sq_thread_park fs/io_uring.c:7142 [inline]
RIP: 0010:io_uring_flush+0x105b/0x1640 fs/io_uring.c:8596
Code: 0f 85 f4 04 00 00 48 8b 44 24 20 48 8b 98 a8 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 8d ab 48 01 00 00 48 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 b9 04 00 00 48 83 bb 48 01 00 00 00 74 37 e8 1c
RSP: 0018:ffffc9000a66fd70 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff81dad5e8
RDX: 0000000000000029 RSI: ffffffff81dad5f5 RDI: ffff888099c7f1a8
RBP: 0000000000000148 R08: 0000000000000000 R09: ffff88809d6443c7
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88809ad1fbc0
R13: 0000000000000000 R14: ffff888099c7f000 R15: 0000000000000000
FS:  00000000020a7940(0000) GS:ffff8880ae400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000768000 CR3: 000000009c90d000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 filp_close+0xb4/0x170 fs/open.c:1276
 __close_fd+0x2f/0x50 fs/file.c:671
 __do_sys_close fs/open.c:1295 [inline]
 __se_sys_close fs/open.c:1293 [inline]
 __x64_sys_close+0x69/0x100 fs/open.c:1293
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x417901
Code: 75 14 b8 03 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 a4 1a 00 00 c3 48 83 ec 08 e8 0a fc ff ff 48 89 04 24 b8 03 00 00 00 0f 05 <48> 8b 3c 24 48 89 c2 e8 53 fc ff ff 48 89 d0 48 83 c4 08 48 3d 01
RSP: 002b:00007ffca0939140 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000000417901
RDX: 0000001b31f20000 RSI: 0000000000000001 RDI: 0000000000000003
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: 00007ffca0939230 R11: 0000000000000293 R12: 000000000118d9c0
R13: 000000000118d9c0 R14: ffffffffffffffff R15: 000000000118cf4c
Modules linked in:
---[ end trace 188a1c353995f688 ]---
RIP: 0010:io_sq_thread_park fs/io_uring.c:7142 [inline]
RIP: 0010:io_uring_flush+0x105b/0x1640 fs/io_uring.c:8596
Code: 0f 85 f4 04 00 00 48 8b 44 24 20 48 8b 98 a8 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 8d ab 48 01 00 00 48 89 ea 48 c1 ea 03 <80> 3c 02 00 0f 85 b9 04 00 00 48 83 bb 48 01 00 00 00 74 37 e8 1c
RSP: 0018:ffffc9000a66fd70 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff81dad5e8
RDX: 0000000000000029 RSI: ffffffff81dad5f5 RDI: ffff888099c7f1a8
RBP: 0000000000000148 R08: 0000000000000000 R09: ffff88809d6443c7
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88809ad1fbc0
R13: 0000000000000000 R14: ffff888099c7f000 R15: 0000000000000000
FS:  00000000020a7940(0000) GS:ffff8880ae400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000004 CR3: 000000009c90d000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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