bcache: fix crash in bcache_btree_node_alloc_fail tracepoint

'b' was NULL.

Change-Id: Icac0fd04afa2d23f213d96d51afd53374e6dd0c0
diff --git a/include/trace/events/bcache.h b/include/trace/events/bcache.h
index 6778e41..981acf7 100644
--- a/include/trace/events/bcache.h
+++ b/include/trace/events/bcache.h
@@ -261,9 +261,9 @@
 	TP_ARGS(b)
 );
 
-DEFINE_EVENT(btree_node, bcache_btree_node_alloc_fail,
-	TP_PROTO(struct btree *b),
-	TP_ARGS(b)
+DEFINE_EVENT(cache_set, bcache_btree_node_alloc_fail,
+	TP_PROTO(struct cache_set *c),
+	TP_ARGS(c)
 );
 
 DEFINE_EVENT(btree_node, bcache_btree_node_free,