btr_page_reorganize_low(): Do not invoke lock_move_reorganize_page()
on a dummy index during change buffer merge. The ibuf.index page
latch that we are holding may block a DDL operation that is waiting
in ibuf_delete_for_discarded_space() while holding exclusive
lock_sys.latch. ibuf_insert_low() would refuse to buffer a change
if any locks exist for the index page.