intrng: Shuffle unhandled interrupts too

When interrupt vectors are first allocated, they get assigned to
CPU #0; at SI_SUB_SMP / SI_ORDER_SECOND (aka once we have multiple
CPUs), the intr_irq_shuffle SYSINIT clears their CPU sets with the
effect of forcing them to be assigned to new CPUs later.

In case where interrupt vectors were allocated *but not yet bound*
this code did not run, with the effect that those interrupts would
remain pinned to CPU #0 forever.  This affected the ena(4) driver,
which allocates interrupts for I/O when the device is attached but
doesn't set them up until the interface is brought up much later in
the boot process (and, crucially, long after intr_irq_shuffle runs).

Adjust intr_irq_shuffle to clear the CPU set for an interrupt source
even if it currently has no handlers, so that it will be properly
assigned to a CPU when it is used later.

Reviewed by:	andrew, mhorne
MFC after:	1 month
Sponsored by:	Amazon
Differential Revision:	https://reviews.freebsd.org/D55284

sys/kern/subr_intr.c

@@ -1291,10 +1291,22 @@ intr_irq_shuffle(void *arg __unused)
 	irq_assign_cpu = true;
 	for (i = 0; i < intr_nirq; i++) {
 		isrc = irq_sources[i];
-		if (isrc == NULL || isrc->isrc_handlers == 0 ||
+		if (isrc == NULL ||
 		    isrc->isrc_flags & (INTR_ISRCF_PPI | INTR_ISRCF_IPI))
 			continue;
 
+		/*
+		 * We can reach this point with isrc_handlers == 0 if a
+		 * driver allocates interrupts but does not set them up
+		 * immediately; for example, a network driver might
+		 * postpone calling bus_setup_intr on I/O IRQ(s) until
+		 * the interface is brought up.
+		 */
+		if (isrc->isrc_handlers == 0) {
+			CPU_ZERO(&isrc->isrc_cpu);
+			continue;
+		}
+
 		if (isrc->isrc_event != NULL &&
 		    isrc->isrc_flags & INTR_ISRCF_BOUND &&
 		    isrc->isrc_event->ie_cpu != CPU_FFS(&isrc->isrc_cpu) - 1)