Kconfig: BLK_THROTTLE -> BLK_DEV_THROTTLING

It would seem that `CONFIG_BLK_THROTTLE' doesn't exist,
as it is only referenced in the documentation for
`CONFIG_BLK_CGROUP'. The only other choice is
`CONFIG_BLK_DEV_THROTTLING':

  $ git grep --cached THROTTL -- \*Kconfig
  block/Kconfig:config BLK_DEV_THROTTLING
  init/Kconfig:   CONFIG_BLK_THROTTLE=y.
Signed-off-by: NMichael Witten <mfwitten@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

init/Kconfig

@@ -745,7 +745,7 @@ config BLK_CGROUP
 	One needs to also enable actual IO controlling logic/policy. For
 	enabling proportional weight division of disk bandwidth in CFQ, set
 	CONFIG_CFQ_GROUP_IOSCHED=y; for enabling throttling policy, set
-	CONFIG_BLK_THROTTLE=y.
+	CONFIG_BLK_DEV_THROTTLING=y.
 
 	See Documentation/cgroups/blkio-controller.txt for more information.