fs: descend into sub directories when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/fs/zfs/Makefile b/fs/zfs/Makefile
index 7090416..fa58b7f 100644
--- a/fs/zfs/Makefile
+++ b/fs/zfs/Makefile
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_CMD_ZFS) := dev.o zfs.o zfs_fletcher.o zfs_sha256.o zfs_lzjb.o
+obj-y := dev.o zfs.o zfs_fletcher.o zfs_sha256.o zfs_lzjb.o