Update mmap to add temporarily page_set_swap until swap is implemented

src/vm/mmap.c

@@ -87,3 +87,21 @@ mmap_cleanup (struct hash_elem *e, void *aux UNUSED)
   file_close (mmap->file);
   free (mmap);
 }
+
+/* Updates the 'owner' thread's page table entry for virtual address 'upage'
+   to have a present bit of 0 and stores the specified swap slot value in the
+   entry for later retrieval from disk. */
+void
+page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
+{
+
+}
+
+/* Given that the page with user address 'upage' owned by 'owner' is flagged
+   to be in the swap disk via the owner's page table, returns its stored
+   swap slot. Otherwise panics the kernel. */
+size_t
+page_get_swap (struct thread *owner, void *upage)
+{
+  return 0;
+}