firewire: core: improve check for local node

My recently added test for a device being local in fw-cdev.c got it
slightly wrong:  Comparisons of node IDs are only valid if the
generation is current, which I forgot to check.  Normally, serialization
by card->lock takes care of this, but a device in FW_DEVICE_GONE state
will necessarily have a wrong generation and invalid node_id.

The "is it local?" check is made 100% correct and simpler now by means
of a struct fw_device flag which is set at fw_device creation.

Besides the fw-cdev site which was to be fixed, there is another site
which can make use of the new flag, and an RFC-2734 driver will benefit
from it too.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/fw-cdev.c

diff --git a/drivers/firewire/fw-cdev.c b/drivers/firewire/fw-cdev.c
index 7eb6594..8a5e6ae2 100644
--- a/drivers/firewire/fw-cdev.c
+++ b/drivers/firewire/fw-cdev.c
@@ -739,15 +739,11 @@
 static int ioctl_add_descriptor(struct client *client, void *buffer)
 {
 	struct fw_cdev_add_descriptor *request = buffer;
-	struct fw_card *card = client->device->card;
 	struct descriptor_resource *r;
 	int ret;
 
 	/* Access policy: Allow this ioctl only on local nodes' device files. */
-	spin_lock_irq(&card->lock);
-	ret = client->device->node_id != card->local_node->node_id;
-	spin_unlock_irq(&card->lock);
-	if (ret)
+	if (!client->device->is_local)
 		return -ENOSYS;
 
 	if (request->length > 256)