ovl: allow distributed fs as lower layer

Allow filesystems with .d_revalidate as lower layer(s), but not as upper
layer.

For local filesystems the rule was that modifications on the layers
directly while being part of the overlay results in undefined behavior.

This can easily be extended to distributed filesystems: we assume the tree
used as lower layer is static, which means ->d_revalidate() should always
return "1".  If that is not the case, return -ESTALE, don't try to work
around the modification.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>