[XFS] Remove Makefile wrappers in XFS

Makefile (and Kbuild) would include Makefile-linux-26 I doubt XFS will
really still compile on 2.4; so drop that. This moves Makefile-linux-26
into Makefile and drops Kbuild. Also having wrappers as both Kbuild and
Makefile seemed redundant anyways.

The patch is relatively large because it renames a file, but no functional
changes.

SGI-PV: 971050
SGI-Modid: xfs-linux-melb:xfs-kern:29781a

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>