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.

*** 70,80 ****
  static int      segnf_getprot(struct seg *seg, caddr_t addr,
                      size_t len, uint_t *protv);
  static u_offset_t segnf_getoffset(struct seg *seg, caddr_t addr);
  static int      segnf_gettype(struct seg *seg, caddr_t addr);
  static int      segnf_getvp(struct seg *seg, caddr_t addr, struct vnode **vpp);
- static void     segnf_dump(struct seg *seg);
  static int      segnf_pagelock(struct seg *seg, caddr_t addr, size_t len,
                      struct page ***ppp, enum lock_type type, enum seg_rw rw);
  
  
  const struct seg_ops segnf_ops = {
--- 70,79 ----
*** 96,106 ****
          .getoffset      = segnf_getoffset,
          .gettype        = segnf_gettype,
          .getvp          = segnf_getvp,
          .advise         = (int (*)(struct seg *, caddr_t, size_t, uint_t))
                  segnf_nop,
-         .dump           = segnf_dump,
          .pagelock       = segnf_pagelock,
  };
  
  /*
   * vnode and page for the page of zeros we use for the nf mappings.
--- 95,104 ----
*** 435,452 ****
  
          *vpp = &nfvp;
          return (0);
  }
  
- /*
-  * segnf pages are not dumped, so we just return
-  */
- /* ARGSUSED */
- static void
- segnf_dump(struct seg *seg)
- {}
- 
  /*ARGSUSED*/
  static int
  segnf_pagelock(struct seg *seg, caddr_t addr, size_t len,
      struct page ***ppp, enum lock_type type, enum seg_rw rw)
  {
--- 433,442 ----