The buf_page_free() call that was introduced in MDEV-15528 was
performed too early in fseg_free_page(), tripping a debug check
in ibuf_remove_free_page(). In all other callers, we can (and will)
invoke buf_page_free() right after fseg_free_page(), but in
ibuf_remove_free_page() we will defer that call to the end of the
mini-transaction. (That call was already present.)