ovl: decode indexed dir file handles

Decoding an indexed dir file handle is done by looking up the file handle
in index dir by name and then decoding the upper dir from the index origin
file handle. The decoded upper path is used to lookup an overlay dentry of
the same path.

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