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: <stable@vger.kernel.org>
Reported-by: NJiang Liu <jiang.liu@huawei.com>
Tested-by: NGaohuai Han <hangaohuai@huawei.com>
Signed-off-by: NMaciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: NDan Williams <djbw@fb.com>

drivers/dca/dca-core.c

@@ -420,6 +420,11 @@ void unregister_dca_provider(struct dca_provider *dca, struct device *dev)
 
 	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);