ktime: Simplify ktime_compare implementation

ktime_sub can be used here instread of two conditional checks.

Signed-off-by: Mariusz Skamra <mariuszx.skamra@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@intel.com>
Link: http://lkml.kernel.org/r/1495803647-9504-1-git-send-email-mariuszx.skamra@intel.com