Found that commit 990e78116d38059c ("block: loop: fix deadlock between open and remove") went to 5.13-rc6.

#syz fix: block: loop: fix deadlock between open and remove

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Now syzbot is reporting

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(nbd_index_mutex);
                               lock(&bdev->bd_mutex);
                               lock(nbd_index_mutex);
  lock(&bdev->bd_mutex);

at https://syzkaller.appspot.com/text?tag=CrashReport&x=11b8a5bbd00000 .