Commit 146d62e5a586 ("ovl: detect overlapping layers") made sure we
don't have overapping layers, but it also broke the arguably valid use
case of
mount -olowerdir=/,upperdir=/subdir,..
where subdir also resides on the root fs.
I also see that we check for a trap at lookup time, so the question is
what does the up-front layer check buy us?
Thanks,
Miklos
