Update mmap to add a get helper function to find a mmap entry from its mapping

src/vm/mmap.h

@@ -18,6 +18,7 @@ struct mmap_entry {
 };
 
 bool mmap_init (struct thread *t);
+struct mmap_entry *mmap_get (mapid_t mapping);
 struct mmap_entry *mmap_insert (struct file *file, void *upage);
 void mmap_destroy (void);