Update mmap to add an insert helper function to allocate and add new mmap entries to the hash table

src/vm/mmap.h

@@ -18,6 +18,7 @@ struct mmap_entry {
 };
 
 bool mmap_init (void);
+struct mmap_entry *mmap_insert (struct file *file, void *upage);
 void mmap_destroy (void);
 
 #endif /* vm/mmap.h */