blk-throttle: implement sq_to_tg(), sq_to_td() and throtl_log()

blk-throttle: implement sq_to_tg(), sq_to_td() and throtl_log()

Now that both throtl_data and throtl_grp embed throtl_service_queue,
we can unify throtl_log() and throtl_log_tg().

* sq_to_tg() is added.  This returns the throtl_grp a service_queue is
  embedded in.  If the service_queue is the top-level one embedded in
  throtl_data, NULL is returned.

* sq_to_td() is added.  A service_queue is always associated with a
  throtl_data.  This function finds the associated td and returns it.

* throtl_log() is updated to take throtl_service_queue instead of
  throtl_data.  If the service_queue is one embedded in throtl_grp, it
  prints the same header as throtl_log_tg() did.  If it's one embedded
  in throtl_data, it behaves the same as before.  This renders
  throtl_log_tg() unnecessary.  Removed.

This change is necessary for hierarchy support as we're gonna be using
the same code paths to dispatch bios to intermediate service_queues
embedded in throtl_grps and the top-level service_queue embedded in
throtl_data.

This patch doesn't make any behavior changes.

v2: throtl_log() didn't print a space after blkg path.  Updated so
    that it prints a space after throtl_grp path.  Spotted by Vivek.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>