time: Change types to new y2038 safe __kernel_itimerspec

timer_set/gettime and timerfd_set/get apis use struct itimerspec at the
user interface layer.  struct itimerspec is not y2038-safe.  Change these
interfaces to use y2038-safe struct __kernel_itimerspec instead.  This will
help define new syscalls when 32bit architectures select CONFIG_64BIT_TIME.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: viro@zeniv.linux.org.uk
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: y2038@lists.linaro.org
Link: https://lkml.kernel.org/r/20180617051144.29756-4-deepa.kernel@gmail.com