use NULL setpagesize segop as a shorthand for ENOTSUP
Instead of forcing every segment driver to implement a dummp function to
return (hopefully) ENOTSUP, handle NULL setpagesize segop function pointer
as "return ENOTSUP" shorthand.

*** 2004,2014 ****
  }
  
  int
  segop_setpagesize(struct seg *seg, caddr_t addr, size_t len, uint_t szc)
  {
!         VERIFY3P(seg->s_ops->setpagesize, !=, NULL);
  
          return (seg->s_ops->setpagesize(seg, addr, len, szc));
  }
  
  int
--- 2004,2015 ----
  }
  
  int
  segop_setpagesize(struct seg *seg, caddr_t addr, size_t len, uint_t szc)
  {
!         if (seg->s_ops->setpagesize == NULL)
!                 return (ENOTSUP);
  
          return (seg->s_ops->setpagesize(seg, addr, len, szc));
  }
  
  int