genirq: Add return value to check_irq_resend()

In preparation for an interrupt injection interface which can be used
safely by error injection mechanisms. e.g. PCI-E/ AER, add a return value
to check_irq_resend() so errors can be propagated to the caller.

Split out the software resend code so the ugly #ifdef in check_irq_resend()
goes away and the whole thing becomes readable.

Fix up the caller in debugfs. The caller in irq_startup() does not care
about the return value as this is unconditionally invoked for all
interrupts and the resend is best effort anyway.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lkml.kernel.org/r/20200306130623.775200917@linutronix.de

kernel/irq/debugfs.c

@@ -218,8 +218,7 @@ static ssize_t irq_debug_write(struct file *file, const char __user *user_buf,
 			err = -EINVAL;
 		} else {
 			desc->istate |= IRQS_PENDING;
-			check_irq_resend(desc);
-			err = 0;
+			err = check_irq_resend(desc);
 		}
 
 		raw_spin_unlock_irqrestore(&desc->lock, flags);