LU-7961 build: Fix ldiskfs source autodetect for CentOS 6

LU-7961 build: Fix ldiskfs source autodetect for CentOS 6

CentOS 6 installed the kernel source in to the following path
which differs slightly from the RHEL version.  Expand the search
pattern to encompass both the CentOS and RHEL paths.

/usr/src/debug/*/linux-2.6.32-x.y.z.el6.centos.plus.x86_64

Change-Id: I0a102148f34d57b94dddfac7819303314c61a376
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-on: http://review.whamcloud.com/19248
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>