6149 use NULL capable segop as a shorthand for no-capabilities

*** 1996,2005 ****
--- 1996,2008 ----
  }
  
  int
  segop_capable(struct seg *seg, segcapability_t cap)
  {
+         if (seg->s_ops->capable == NULL)
+                 return (0);
+ 
          return (seg->s_ops->capable(seg, cap));
  }
  
  int
  segop_inherit(struct seg *seg, caddr_t addr, size_t len, uint_t op)