Update start_process to acquire filesys lock when loading user process file

src/lib/kernel/list.c

@@ -170,6 +170,7 @@ list_insert (struct list_elem *before, struct list_elem *elem)
 {
   ASSERT (is_interior (before) || is_tail (before));
   ASSERT (elem != NULL);
+ 
   // Sanity checks to prevent (some) loop lists
   ASSERT (before != elem);
   ASSERT (before->prev != elem);