NFS mounts can be disconnected from fs root.  Don't fail the overlapping
layer check because of this.

The check is not authoritative anyway, since topology can change during or
after the check.

Reported-by: Antti Antinoja <antti@fennosys.fi> 
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Fixes: 146d62e5 ("ovl: detect overlapping layers")