ovl: fix dentry reference leak after changes to underlying layers

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>