LU-9019 libcfs: add ktime_compare for platforms lacking it.

LU-9019 libcfs: add ktime_compare for platforms lacking it.

The function ktime_compare() was added to the linux kernel
starting with verison 3.19. Add to libcfs support for older
platforms lacking this function.

Test-Parameters: trivial

Change-Id: I7826b8e78d0dc2c633490a2949210176a0003d9a
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/30475
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>