ovl: wire up NFS export operations

Now that NFS export operations are implemented, enable overlayfs NFS
export support if the "nfs_export" feature is enabled.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -1331,6 +1331,9 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 		}
 	}
 
+	if (ofs->config.nfs_export)
+		sb->s_export_op = &ovl_export_operations;
+
 	/* Never override disk quota limits or use reserved space */
 	cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);