posix-stubs: Use get_timespec64() and put_timespec64()

This is a follow-up to commit 5c499410 ("posix-timers: Use
get_timespec64() and put_timespec64()"), which left two system call using
copy_from_user()/copy_to_user().

Change them as well for consistency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: y2038@lists.linaro.org
Cc: John Stultz <john.stultz@linaro.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Deepa Dinamani <deepa.kernel@gmail.com>
Link: https://lkml.kernel.org/r/20171013183009.3442318-1-arnd@arndb.de