LU-3462 lnet: Fix path to ldiskfs module

LU-3462 lnet: Fix path to ldiskfs module

After this commit:

4dd31ba LU-3462 ldiskfs: Subsume ldiskfs's build system into lustre

The path to the ldiskfs module in lnet/utils/debug.c is updated
to point to the new location (one directory higher).

Change-Id: I949a9cdad107755f66d10dafaf0e8692587e902e
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/7639
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Hudson
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>