genirq/chip: Rework handle_fasteoi_mask_irq()

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

Note: The mask_irq() operation in the second condition was redundant as the
interrupt is already masked right at the beginning of the function.

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/20250429065421.175652864@linutronix.de