cls/rgw: trim all usage entries in cls_rgw

cls/rgw: trim all usage entries in cls_rgw

Currently trim usage will only trim upto 128 omap entries, since we need
to run this in a loop until we're done, actually make the cls return
-ENODATA so that we know when to stop the loop (inspired by a similar
call in cls_log) this involves the following changes

* return -ENODATA when iterate entries goes through and the value of
  iter (which is set as the last value of key when succeeded)
* use IoCtx for calling the loop from within cls rather than in rgw
* drop the goto call in rgw_rados since we can return once we're done
  processing

Fixes: http://tracker.ceph.com/issues/22234
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit b548a3f3443452210d92cad574bcb73ba6d2ce42)