syzbot excercised the forbidden practice of moving the workdir under
lowerdir while overlayfs is mounted and tripped a dentry reference leak.

Fixes: c63e56a4 ("ovl: do not open/llseek lower file with upper sb_writers held")
Reported-and-tested-by: syzbot+8608bb4553edb8c78f41@syzkaller.appspotmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>