genirq/spurious: Switch to lock guards

Convert all lock/unlock pairs to guards and tidy up the code.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20250429065420.497714413@linutronix.de