ktime: Provide typesafe ktime_to_ns()

Using ktime_to_ns() is nice to help backports to stable kernels.

Having a typesafe function instead of a macro avoid stupid typos
and waste of time tracking these typos.
Reported-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/20180711181641.10369-1-edumazet@google.com

include/linux/ktime.h

@@ -93,8 +93,11 @@ static inline ktime_t timeval_to_ktime(struct timeval tv)
 /* Map the ktime_t to timeval conversion to ns_to_timeval function */
 #define ktime_to_timeval(kt)		ns_to_timeval((kt))
 
-/* Convert ktime_t to nanoseconds - NOP in the scalar storage format: */
-#define ktime_to_ns(kt)			(kt)
+/* Convert ktime_t to nanoseconds */
+static inline s64 ktime_to_ns(const ktime_t kt)
+{
+	return kt;
+}
 
 /**
  * ktime_compare - Compares two ktime_t variables for less, greater or equal