ibuf_reset_free_bits(): Remove, as there already is a similar function
ibuf_reset_free_bits_with_type().

ibuf_reset_free_bits_with_type(), ibuf_set_free_bits(),
ibuf_update_free_bits_if_full(), btr_leaf_page_release(),
buf_page_make_young(): Replace page_t with buf_block_t.

btr_compress(): Replace btr_page_get() with btr_block_get().