Btrfs: cleaner make clean

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
index 5af2426..a4e2df6 100644
--- a/fs/btrfs/Makefile
+++ b/fs/btrfs/Makefile
@@ -18,5 +18,5 @@
 all::
 	$(MAKE) -C $(KERNELDIR) M=`pwd` modules
 clean::
-	rm *.o btrfs.ko
+	$(MAKE) -C $(KERNELDIR) M=`pwd` clean
 endif