LU-3173 build: Incorrect detection of ldiskfs series to use

LU-3173 build: Incorrect detection of ldiskfs series to use

Extract correct upstream kernel release for RHEL kernels and
form it into RHEL_KERNEL_VERSION as
$(VERSION).$(PATCHLEVEL).$(SUBLEVEL)-$(RHEL_RELEASE)
Then use this string for correct ldiskfs series selection.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Idda0a4b02a5090b7be6fb81b8f98175559e575aa
Reviewed-on: http://review.whamcloud.com/6056
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>