eventpoll: s/0/NULL/ in pointer context

Index: linux-2.6.10-bk11-warnings/fs/eventpoll.c
===================================================================
@@ -806,7 +806,7 @@
 	 * write-holding "sem" we can be sure that no file cleanup code will hit
 	 * us during this operation. So we can avoid the lock on "ep->lock".
 	 */
-	while ((rbp = rb_first(&ep->rbr)) != 0) {
+	while ((rbp = rb_first(&ep->rbr)) != NULL) {
 		epi = rb_entry(rbp, struct epitem, rbn);
 		ep_remove(ep, epi);
 	}