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.

@@ -1987,11 +1987,12 @@
 }
 
 void
 segop_dump(struct seg *seg)
 {
-        VERIFY3P(seg->s_ops->dump, !=, NULL);
+        if (seg->s_ops->dump == NULL)
+                return;
 
         seg->s_ops->dump(seg);
 }
 
 int