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

*** 3520,3536 ****
          seg = as_segat(as, addr);
          if (seg == NULL) {
                  AS_LOCK_EXIT(as, &as->a_lock);
                  return (EFAULT);
          }
-         /*
-          * catch old drivers which may not support getmemid
-          */
-         if (seg->s_ops->getmemid == NULL) {
-                 AS_LOCK_EXIT(as, &as->a_lock);
-                 return (ENODEV);
-         }
  
          sts = segop_getmemid(seg, addr, memidp);
  
          AS_LOCK_EXIT(as, &as->a_lock);
          return (sts);
--- 3520,3529 ----