The unionfs is a Linux filesystem service which implements a union mount for Linux file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.
Download (HTTP): http://download.filesystems.org/unionfs/unionfs-2.x/
Download (FTP): ftp://ftp.filesystems.org/pub/unionfs/unionfs-2.x
Unionfs consists of a kernel patch. Choose the version closest to your kernel version.
Now, cd to your kernel source top level directory and run the following command:
gunzip -c <location-of-unionfs-patch> | patch -p1
Where "location-of-unionfs-patch" is the path to your unionfs-2.x.diff.gz.