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: Michael Witten <mfwitten@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>