block: make CONFIG_BLK_CGROUP visible

Make the config visible, so we can choose from CONFIG_BLK_CGROUP=y
and CONFIG_BLK_CGROUP=m when CONFIG_IOSCHED_CFQ=m.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/Kconfig b/block/Kconfig
index e20fbde..f9e89f4 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -78,8 +78,9 @@
 	Protection.  If in doubt, say N.
 
 config BLK_CGROUP
-	bool
+	tristate "Block cgroup support"
 	depends on CGROUPS
+	depends on CFQ_GROUP_IOSCHED
 	default n
 	---help---
 	Generic block IO controller cgroup interface. This is the common