rgw: fix memory growth while deleteing objects with

rgw: fix memory growth while deleteing objects with
radosgw-admin bucket rm --bucket=$BIG_BUCKET --bypass-gc --purge-objects
by freeing <rgw_obj, RGWObjState> map elements allocated at
https://github.com/ceph/ceph/blob/master/src/rgw/rgw_rados.cc#L236
  result = &objs_state[obj];

Fixes: https://tracker.ceph.com/issues/40700
Signed-off-by: Mark Kogan <mkogan@redhat.com>
(cherry picked from commit 21560ed2e141137bbcb4d871eb3908d293a83948)

Conflicts:
    src/rgw/rgw_bucket.cc
- in mimic, "invalidate" is a method of "obj_ctx.obj" where in master it's
  a method of the parent class "obj_ctx"