btr_check_node_ptr(): Replace page_t* parameter with buf_block_t*.

btr_free_externally_stored_field(): Add const qualifier to rec.
Remove an explicit buf_block_align() call, but replace an
mtr_memo_contains() with mtr_memo_contains_page().

row_upd_rec_sys_fields(): Reorder an assertion containing buf_block_align()
so that the costly call can be avoided in some cases.