ovl: index directories on copy up for NFS export

With the NFS export feature enabled, all dirs are indexed on copy up.
Non-dir files are copied up directly to indexdir and then hardlinked
to upper dir.

Directories are copied up to indexdir, then an index entry is created
in indexdir with 'upper' xattr pointing to the copied up dir and then
the copied up dir is moved to upper dir.

Directory index is also used for consistency verification, like
detecting multiple redirected dirs to the same lower dir on lookup.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>