ktime: Provide typesafe ktime_to_ns()

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