when buffered changes are to be discarded

ibuf_add_free_page(): Lower the latching order of the newly allocated page
to SYNC_IBUF_TREE_NODE_NEW after latching the insert buffer tree root.

This bug always was bogus UNIV_SYNC_DEBUG alarm. The function
buf_block_dbg_add_level() is a no-op unless UNIV_SYNC_DEBUG is defined.