igb: Fix potential invalid memory access in igb_init_module()

The pci_register_driver() can fail and when this happened, the dca_notifier
needs to be unregistered, otherwise the dca_notifier can be called when
igb fails to install, resulting to invalid memory access.

Fixes: bbd98fe ("igb: Fix DCA errors and do not use context index for 82576")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

Author: Yuan Can

drivers/net/ethernet/intel/igb/igb_main.c

@@ -637,6 +637,10 @@ static int __init igb_init_module(void)
 	dca_register_notify(&dca_notifier);
 #endif
 	ret = pci_register_driver(&igb_driver);
+#ifdef CONFIG_IGB_DCA
+	if (ret)
+		dca_unregister_notify(&dca_notifier);
+#endif
 	return ret;
 }