btrfs: don't try to notify udev about missing devices

If we remove a missing device, bdev is null, and if we
send that off to btrfs_kobject_uevent we'll panic.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 5cce6aa..485a542 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -1556,7 +1556,8 @@
 	ret = 0;
 
 	/* Notify udev that device has changed */
-	btrfs_kobject_uevent(bdev, KOBJ_CHANGE);
+	if (bdev)
+		btrfs_kobject_uevent(bdev, KOBJ_CHANGE);
 
 error_brelse:
 	brelse(bh);