genirq/irqdesc: Switch to lock guards

Replace all lock/unlock pairs with lock guards and simplify the code flow.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/871ptaqhoo.ffs@tglx