timers/migration: Read childmask and parent pointer in a single place

timers/migration: Read childmask and parent pointer in a single place

Reading the childmask and parent pointer is required when propagating
changes through the hierarchy. At the moment this reads are spread all over
the place which makes it harder to follow.

Move those reads to a single place to keep code clean.

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/20240716-tmigr-fixes-v4-5-757baa7803fe@linutronix.de