timekeeping: Make do_adjtimex() reusable

Split out the actual functionality of adjtimex() and make do_adjtimex() a
wrapper which feeds the core timekeeper into it and handles the result
including audit at the call site.

This allows to reuse the actual functionality for auxiliary clocks.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20250625183758.187322876@linutronix.de