[KTIME]: Introduce ktime_us_delta

This provides a reusable time difference function which returns the difference in
microseconds, as often used in the DCCP code.

Commiter note: renamed ktime_delta to ktime_us_delta and put it in ktime.h.
Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: NArnaldo Carvalho de Melo <acme@ghostprotocols.net>

include/linux/ktime.h

@@ -279,6 +279,11 @@ static inline s64 ktime_to_us(const ktime_t kt)
 	return (s64) tv.tv_sec * USEC_PER_SEC + tv.tv_usec;
 }
 
+static inline s64 ktime_us_delta(const ktime_t later, const ktime_t earlier)
+{
+       return ktime_to_us(ktime_sub(later, earlier));
+}
+
 /*
  * The resolution of the clocks. The resolution value is returned in
  * the clock_getres() system call to give application programmers an