dca: check against empty dca_domains list before unregister provider

When providers get blocked unregister_dca_providers() is called ending up
with dca_providers and dca_domain lists emptied. Dca should be prevented from
trying to unregister any provider if dca_domain list is found empty.

Cc: <[e-mail]>
Reported-by: Jiang Liu <[e-mail]>
Tested-by: Gaohuai Han <[e-mail]>
Signed-off-by: Maciej Sosnowski <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/dca/dca-core.c

···
 
 	raw_spin_lock_irqsave(&dca_lock, flags);
 
+	if (list_empty(&dca_domains)) {
+		raw_spin_unlock_irqrestore(&dca_lock, flags);
+		return;
+	}
+
 	list_del(&dca->node);
 
 	pci_rc = dca_pci_rc_from_dev(dev);