btrfs: eliminate insert label in add_falloc_range

By way of inverting the list_empty conditional the insert label can be
eliminated, making the function's flow entirely linear.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>