btrfs: move ref_mod modification into the if (ref) logic

We only use this logic if our ref isn't a ref_head, so move it up into
the if (ref) case since we know that this is a normal ref and not a
delayed ref head.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>