bcache: fix crash in bcache_btree_node_alloc_fail tracepoint

'b' was NULL.

Change-Id: Icac0fd04afa2d23f213d96d51afd53374e6dd0c0
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 39c7f5b..f823785 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1096,7 +1096,7 @@
 err:
 	mutex_unlock(&c->bucket_lock);
 
-	trace_bcache_btree_node_alloc_fail(b);
+	trace_bcache_btree_node_alloc_fail(c);
 	return b;
 }