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.

@@ -3520,17 +3520,10 @@
         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);