page_mem_free(): Define in the same file with the only caller
page_cur_delete_rec().

page_dir_slot_set_rec(): Add const qualifier to a parameter.

page_dir_delete_slot(): Merge with the only caller page_dir_balance_slot().

page_dir_add_slot(): Merge with the only caller page_dir_split_slot().

page_dir_split_slot(), page_dir_balance_slot(): Define in the
same compilation unit with the callers, and simplify the code.