genirq: Introduce IRQD_SINGLE_TARGET flag

Many interrupt chips allow only a single CPU as interrupt target. The core
code has no knowledge about that. That's unfortunate as it could avoid
trying to readd a newly online CPU to the effective affinity mask.

Add the status flag and the necessary accessors.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Link: http://lkml.kernel.org/r/20170619235447.352343969@linutronix.de

kernel/irq/debugfs.c

@@ -105,6 +105,7 @@ static const struct irq_bit_descr irqdata_states[] = {
 	BIT_MASK_DESCR(IRQD_PER_CPU),
 	BIT_MASK_DESCR(IRQD_NO_BALANCING),
 
+	BIT_MASK_DESCR(IRQD_SINGLE_TARGET),
 	BIT_MASK_DESCR(IRQD_MOVE_PCNTXT),
 	BIT_MASK_DESCR(IRQD_AFFINITY_SET),
 	BIT_MASK_DESCR(IRQD_SETAFFINITY_PENDING),