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

src/vm/mmap.c

@@ -15,6 +15,21 @@ mmap_init (struct thread *t)
   return hash_init (&t->mmap_files, mmap_hash, mmap_less, NULL);
 }
 
+struct mmap_entry *
+mmap_get (mapid_t mapping)
+{
+  struct mmap_entry fake_mmap_entry;
+  fake_mmap_entry.mapping = mapping;
+
+  struct hash_elem *e
+    = hash_find (&thread_current ()->mmap_files, &fake_mmap_entry.elem);
+
+  if (e == NULL)
+    return NULL;
+
+  return hash_entry (e, struct mmap_entry, elem);
+}
+
 /* Inserts a new mmap entry into the mmap table for the current thread. Upage
    is the start address of the file data in the user VM. */
 struct mmap_entry *

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);