bcache: fix crash in bcache_btree_node_alloc_fail tracepoint

'b' was NULL.

Change-Id: Icac0fd04afa2d23f213d96d51afd53374e6dd0c0

drivers/md/bcache/btree.c

@@ -1096,7 +1096,7 @@ struct btree *__bch_btree_node_alloc(struct cache_set *c, struct btree_op *op,
 err:
 	mutex_unlock(&c->bucket_lock);
 
-	trace_bcache_btree_node_alloc_fail(b);
+	trace_bcache_btree_node_alloc_fail(c);
 	return b;
 }
 

include/trace/events/bcache.h

@@ -261,9 +261,9 @@ DEFINE_EVENT(btree_node, bcache_btree_node_alloc,
 	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,