Implement syscall for seek w/ S.

src/userprog/syscall.c

@@ -249,9 +249,15 @@ syscall_write (int fd, const void *buffer, unsigned size)
 }
 
 static void
-syscall_seek (int fd UNUSED, unsigned position UNUSED)
+syscall_seek (int fd, unsigned position)
 {
-  //TODO
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info != NULL)
+  {
+    lock_acquire (&filesys_lock);
+    file_seek (file_info->file, position);
+    lock_release (&filesys_lock);
+  }
 }
 
 static unsigned