The header file will be used more widely with the pending cgroup
writeback support and the current set of dummy declarations aren't
enough to handle different config combinations.  Update as follows.

* Drop the struct cgroup declaration.  None of the dummy defs need it.

* Define blkcg as an empty struct instead of just declaring it.

* Wrap dummy function defs in CONFIG_BLOCK.  Some functions use block
  data types and none of them are to be used w/o block enabled.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>