Btrfs: remove BUG() in add_data_reference

Now that we have a helper to report invalid value of extent inline ref
type, we need to quit gracefully instead of throwing out a kernel panic.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>