overlayfs: overlapping upperdir path

[Miklos Szeredi <miklos@xxxxxxxxxx>]

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