Add a mmap destroy function to cleanup all mmap hash table entries upon thread exit

src/vm/mmap.c

@@ -33,3 +33,21 @@ mmap_less (const struct hash_elem *a_, const struct hash_elem *b_,
 
   return a->mapping < b->mapping;
 }
+
+/* Cleans up the mmap table for the current thread. Frees the memory allocated
+   for the mmap entry. */
+static void
+mmap_cleanup (struct hash_elem *e, void *aux UNUSED)
+{
+  struct mmap_entry *mmap = hash_entry (e, struct mmap_entry, elem);
+  file_close (mmap->file);
+  free (mmap);
+}
+
+/* Destroys the mmap table for the current thread. Frees all the memory
+   allocated for the mmap entries. */
+void
+mmap_destroy (void)
+{
+  hash_destroy (&thread_current ()->mmap_files, mmap_cleanup);
+}