genirq/chip: Rework handle_nested_irq()

Use the new helpers to decide whether the interrupt should be handled and
switch the descriptor locking to guard().

Fixup the kernel doc comment while at it.

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/20250429065420.744042890@linutronix.de