Btrfs: fix btrfs_ioctl_dev_info() crash on missing device

When a filesystem is mounted with the degraded option, it is
possible that some of the devices are not there.
btrfs_ioctl_dev_info() crashs in this case because the device
name is a NULL pointer. This ioctl was only used for scrub.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>

fs/btrfs/ioctl.c

@@ -2262,7 +2262,10 @@ static long btrfs_ioctl_dev_info(struct btrfs_root *root, void __user *arg)
 	di_args->bytes_used = dev->bytes_used;
 	di_args->total_bytes = dev->total_bytes;
 	memcpy(di_args->uuid, dev->uuid, sizeof(di_args->uuid));
-	strncpy(di_args->path, dev->name, sizeof(di_args->path));
+	if (dev->name)
+		strncpy(di_args->path, dev->name, sizeof(di_args->path));
+	else
+		di_args->path[0] = '\0';
 
 out:
 	if (ret == 0 && copy_to_user(arg, di_args, sizeof(*di_args)))