timers: Split next timer interrupt logic

Split the logic for getting next timer interrupt (no matter of recalculated
or already stored in base->next_expiry) into a separate function named
next_timer_interrupt(). Make it available to local call sites only.

No functional change.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240221090548.36600-10-anna-maria@linutronix.de