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>

block/Kconfig

@@ -78,8 +78,9 @@ config BLK_DEV_INTEGRITY
 	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