use NULL capable segop as a shorthand for no-capabilities
Instead of forcing every segment driver to implement a dummy "return 0"
function, handle NULL capable segop function pointer as "no copabilities
supported" shorthand.

@@ -2029,11 +2029,12 @@
 }
 
 int
 segop_capable(struct seg *seg, segcapability_t cap)
 {
-        VERIFY3P(seg->s_ops->capable, !=, NULL);
+        if (seg->s_ops->capable == NULL)
+                return (0);
 
         return (seg->s_ops->capable(seg, cap));
 }
 
 int