timekeeping: add arch_offset hook to ktime_get functions

timekeeping: add arch_offset hook to ktime_get functions

commit d004e024058a0eaca097513ce62cbcf978913e0a upstream.

ktime_get and ktime_get_ts were calling timekeeping_get_ns()
but later they were not calling arch_gettimeoffset() so architectures
using this mechanism returned 0 ns when calling these functions.

This happened for example when running Busybox's ping which calls
syscall(__NR_clock_gettime, CLOCK_MONOTONIC, ts) which eventually
calls ktime_get. As a result the returned ping travel time was zero.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>