Btrfs: add modules_install target

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/Makefile

@@ -18,6 +18,8 @@ else
 KERNELDIR := /lib/modules/`uname -r`/build
 all::
 	$(MAKE) -C $(KERNELDIR) M=`pwd` modules
+modules_install::
+	$(MAKE) -C $(KERNELDIR) M=`pwd` modules_install
 clean::
 	$(MAKE) -C $(KERNELDIR) M=`pwd` clean
 endif