use NULL dump segop as a shorthand for no-op
Instead of forcing every segment driver to implement a dummy function that
does nothing, handle NULL dump segop function pointer as a no-op shorthand.

*** 1987,1997 ****
  }
  
  void
  segop_dump(struct seg *seg)
  {
!         VERIFY3P(seg->s_ops->dump, !=, NULL);
  
          seg->s_ops->dump(seg);
  }
  
  int
--- 1987,1998 ----
  }
  
  void
  segop_dump(struct seg *seg)
  {
!         if (seg->s_ops->dump == NULL)
!                 return;
  
          seg->s_ops->dump(seg);
  }
  
  int