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>