hrtimer: Use hrtimer_cb_get_time() helper

Various other helpers contain open-coded implementations of
hrtimer_cb_get_time(). This prevents refactoring the implementation.

Reuse the existing helper.

For this to work, move hrtimer_cb_get_time() a bit up in the file and also
make its argument 'const'.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20250821-hrtimer-cleanup-get_time-v2-7-3ae822e5bfbd@linutronix.de