posix-timers: Use new ktime_get_*_ts64() helpers

Some of the oddly named time accessor functions now have a more consistent
naming, which should be used from now on so the aliases can be removed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: y2038@lists.linaro.org
Cc: Deepa Dinamani <deepa.kernel@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lkml.kernel.org/r/20180618143246.3865099-1-arnd@arndb.de