[ Upstream commit 9179c21dc6ed1c993caa5fe4da876a6765c26af7 ]

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: NAntti Antinoja <antti@fennosys.fi>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Fixes: 146d62e5a586 ("ovl: detect overlapping layers")
Signed-off-by: NSasha Levin <sashal@kernel.org>