ntp: Move timex validation to timekeeping do_adjtimex call.

Move logic that does not need the ntp state to be done
in the timekeeping do_adjtimex() call.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>